To date, we've always used just RPGNow for sales of our products in digital format (PDF). I'm looking into the possibility of adding another digital vendor or two, in order to put our digital offerings in front of more potential customers. So I'm wondering, what do our fans think of this idea? Are there any people reading here who haven't bought our PDFs, but might if they were available from more websites?
I've registered an account on RPGNow to get past supplements, so I would probably not bother registering an account with another vendor now, but don't let me stop you from getting more exposure. 
(I had almost forgotten there were other forums here than for Ars Magica. 
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I'll second that. Especially the last comment. 
I'm a long time customer of RPGNow, and of DriveThruRPG (ever since they removed DRM). I see no problem with you expanding into other stores, and I can't see how anyone will. The only possible problem I can see is if the revenues don't cover the costs of entry, but that's a problem on your end, not mine.
Well, I think we will try Steve Jackson's e23. Obviously we already have a good relationship with Warehouse 23, since they handle all of our mail order/web sales for print books. e23 actually takes a smaller cut than RPGNow, so even if we divided up existing sales and didn't add any new customers with the new outlet, that would at least pay for the cost of giving a second storefront a try.
A big issue with pdf's being the ease of them being dispersed illegally, I would recommend looking at the site DriveThruRpg which marks each pdf with the account name that bought the pdf. So if they turn up on the web you know who it was who did it, and can get them prosecuted.
And on another point, if you could release as many more Ars Magica products as possible on pdf cos I want them all.
I'll also join the choir of seconding that thought.
I'm also registered at e23 - to mail order the printed material - so wouldn't mind to buy all my Ars there. A PDF of reprints of the core rules would be much appreciated - but I understand if security/revenue reasons might not easily allow it. Marking the PDF might be a reasonable solution.