I didn't see an existing thread for this, so here's a pointer to a very important announcement over on the Atlas-Games blog:
Thank you David. Incredible work.
OMG
Took a few days to sink in.
Ars has been art to play & read. You have produced a unique style of play that has endured over years and years and am sure will go on for quite some time.
Sometimes less is more so simply said: Thank-you Mr. Chart!
W
I have read David's post, and commented on the blog, but I just want to say a few words.
David has always gone above and beyond the call of duty. His boundless enthusiasm, determined eye for detail and strong vision of what he wanted from the line are known to all us authors, and I guess the fans. When he returned to the UK for infrequent vacations he took time out of seeing folks to come and stay at Grand Tribunal UK: that is selfless, and I'm not sure I ever said how much it means to us to have him there answering questions and playing games.
Every book was as far as I know a concept originating with David, barring Apprentices; he commissioned us writers to produce the content, and dealt with our occasional disagreements diplomatically and with great wisdom. His calm, sensible hand at the rudder steered Ars all the way, and he showed great discernment and made excellent choices throughout. He assembled an incredibly talented team of writers, and me, who he cited as a dreadful example of what not to do to them on many occasions, often in my hearing! 
So David is a great leader, editor, historian and classicist. I have met a few, mainly in this community. Beyond that however, David is an exceptionally kind, humane and decent kind of cove. Now I'm not the easiest person to manage, what with noose and rabbit hut related injuries, a couple of life threatening illnesses and plenty of odd events making my days interesting over the last decade - but David was always supportive, helpful, and while no doubt rolling his eyes he kept giving me work and believing in me. And I kept writing, and worked some very, very long hours to make sure I did the best I could, and most importantly I learned from David never to write more than your ascribed word count, unlike all those other villainous authors
Writing for Ars has been for a decade my main income - and I have not exactly been rich, but I have loved it. At times David has helped me financially, organising payments in advance, giving of his own money, or paying for Grand Tribunal even in years he could not come. He has listened to my dreadful anecdotes, met my cats and harem of concubines, and been there as not just a boss but as a friend.
When I first heard the news, a short while* before it became public, I tried to talk David in to staying, because I remain delighted by what he and the authors have constructed. I understand fully what he means though: he revolutionised the game in 4th ed I feel, and 5th ed is just better in every respect. Now when fighting after the pubs kick out on Cheltenham High Street I don armour and carry a polearm: under 4th edition it was naked with a stick all the way! 
And so, apart from sharply reducing how often I am arrested, David has been an exceptional figure in the history of gaming: one I feel never actually made a bad decision over a decade. 
I think Ars Magica 5th edition will still be being played for many years to come, and David's legacy to the world of rpg, far greater than just Ars of course, will be long celebrated.
Bravo David. A sad day, but as I always say "lest one good custom should corrupt the world"... Muto Imaginem I guess, but the substance endures... 
cj x
- in Hermetic terms
I would just like to add my alf shukr (1000 thanks) to David for all his hard work in overseeing what I think is the best edition of AM yet! He was always immensely supportive of me while I was writing for the game, even including still speaking to me after I had to pull out of CatC during the revision stages.
You will be greatly missed, Sir, but the very best of luck with your future endeavours. May your road through the desert always be clear and your camel never spit in your ear...
I already commented on the announcement itself, but I agree that David's tenure as line-editor has been a great time to be an Ars Magica fan!
Big news. Good luck, David, and thanks for being a great line editor.
Fifth edition has been an enormous achievement, of which I hope David is proud.
In our culture we value the novel above all other forms of writing, and so novels are the things which get fandoms and movies made. I'd like to defend 5th edition, though, as something which stands with other great book series, and can claim to be better designed, and far better written, than many of the novel series which get raved about.
As a work of fiction, in the sense that it builds a setting, Mythic Europe as David built it can stand with big names like Westeros and the Galactic Empire, and the only reason people don't notice this is because of this bias, in our culture, toward a particular narrative form.
I'd like to congratulate David of finishing what he started, which really is a masterpiece, and wish him all the best in the things he'll go on to do from here.
Thank you, everyone.
I put most things in my letter, but there's one other thing I want to mention here. The Ars Magica community is really good at keeping quiet when asked. I told the authors nearly three years ago, when I solicited the final book in the line. I told the playtesters eighteen months ago, when it first went out for playtest. I told all the attendees at Grand Tribunal UK last summer. Everybody kept it quiet, and I am very grateful, but not surprised.
Dear David,
no 'morituri' around here, I hope. But please think of us Ars fans with forebearance.
And all the best wishes for the future!
Ars Magica 5th edition was definitely one of the few RPG good enough to hook me. I have to thank you, David Chart, for bringing it to life and finding dedicated authors to fill the corners of Mythic Europe.
I hope you will allow us to discover different universes and stories in the future. No matter what new horizons you are sailing into, I wish you the very best.
David, a lot of hugs and thanks for all your work.
Well this was a bit of a bummer, the game has been clearly more "reliable" and quality under David.
Well David, thanks for the years and i wish you well for the future.
Yes, Thank you David, and I agree with what everyone here has said to you in the posts above.
I would also like to join the thanking of David Chart for his great contributions to Ars Magica, making it into IMHO one of the best games ever. I've been fascinated with Ars Magica since 2nd ed but never actually got round to playing it until 4th ed.
David has been an invaluable help and support for me during my relatively short time as author. Being neither an academic, a trained writer, nor having English as my first language I'm sure this has not been the easiest task. I'd like to thank him for all his help and guidance, and for the opportunity to write for the line. It has been time consuming, difficult at times, but great fun and very rewarding on a personal level. I'm glad I accepted the challenge of sending a proposal for the first open call, which opened the door for me.
I've had the opportunity to write and cooperate with some great authors and very nice people. Had I not been involved in this way I may never have visited GTUK at all. But now I've met David on two occasions, met several of the other authors, and I've met a good deal of other great people as well. While David's retirement initially came as a shock, and is also sad, his reasoning is sound and I understand it.
Thank you David, for your great achievements for Ars Magica and for roleplaying in general. Please don't disappear from the community.
(and also thanks to CJ for his nice words, as well as ramming the delightful image into my mind of him buck-naked in Cheltenham)
As Christian touched on, the legacy David leaves is not just one of a superb much-loved game line with some of the highest quality work in the business, but is also one of new opportunities granted and new friendships forged.
I've contributed to a dozen books for Ars Magica, an opportunity given to me by David. I've also contributed outside of the Ars Magica world, something that I simply could not have done without David giving me that initial chance. But it's not just the initial invite to the author pool. That doesn't guarantee a shot at stardom (not sure I've quite made it to stardom, but still). Every book I've pitched for has essentially been me taking a wild swing in the dark, often without quite having a game plan or an exit line. I'm sure that often comes through in my chapters, how could it not, but I remember my initial pitch for The Great Tower in Hermetic Projects being... Well, it wasn't what was required. Other pitches suddenly became three times more work than anticipated as soon as the writing process began. But the engine created by David, what with the author list and the playtesting, on all these books was so strong that it got me through.
The support I received working on Ars Magica is also what gave me the courage, along with Ben McFarland, to take on Sub Rosa when that seemed to have been lost to us and he's been just as supportive of our efforts there too. And coming into the author pool exposed me to a host of people I have huge affection and respect for, people I'm very pleased to call friends beyond the confines of the gaming table and beyond the confines even of individual book projects.
David giving me a shot way back during the Magi of Hermes open call has expanded my horizons beyond my expectations. The opportunity to contribute, the opportunity to develop, and the opportunity to create new stuff and to have people read, enjoy, and use that new stuff has been incredibly rewarding. If I never get the chance to contribute to the Ars Magica line again I'm still hugely grateful for the chance to do so in David's incredible Fifth Edition.
Of course we're all hungry for more, of course we're all now looking forward to editions six and onward, but take a moment to go look over that collection of books on your shelf. From first to last (and the last is set to be something very special) the depth, breadth, and quality of the setting, the rules, the writing, the ideas is second to none. That's a tough act to follow. Just like the man himself.
So long, and thanks for all the fish! 
I guess that, combined with the prospect of more news on Thursday, basically confirms Ars Magica 6th Edition ... or the cancellation of the line altogether. Either way, I'm not happy about it. 
David has been extremely patient with the staff at Atlas Games during production of each book, especially me in the past year. He is completely without peer in this business for what he's done for ArM. All compliments are well deserved.
It's kind of sad news, in my contacts with David he had been unerringly polite and helpful. However, he goes out on a high: how many can claim responsibility for the best gaming world ever? He leaves on his own terms at his own time, his work complete. Well done.
Godspeed David. You did good. Don't be a stranger.