My question is, can one cast Gift of Bear's Fortitude on themselves and then transform themselves into an animal form and have the Soaks from both the spell and the animal form stack together as long as the durations overlap?
Further, could one start with Doublet of Impenetrable Silk, then Bear's Fortitude, then transform and expect all Soaks to stack while durations overlap?
The Bjornaer section of Mystery cults is pretty clear that while transformed into an animal by a spell you are still subject to corpus spells.
So I think I'd allow gift of the Bear's fortidude to continue to raise soak even if the target was changed into a bear.
I wouldn't allow gift of the bear's fortitude to stack with a similar MuAn spell, that would (IMO) be redundant.
Doublet of inpenitrable silk is changing the clothing so I don't see any problem with it stacking along with gift of the bear's fortitude. In order for it to work with a tranformation spell however the animal form would actually need to be wearing the doublet in question.
Images of Circus Bears comes to mind. Put a cap on him and my Magus will hire him for the local fair! 
I wouldn't let Gift of the Bear's Fortitude stack with changing into a beast which has, say, the "Tough Hide" quality (HoH:MC), because that's arguably what the spell already confers.
Na, bring him into the lab and make him into your familiar!
I think that one could go either way...depends on weather you see the spell as making the skin tougher, or actually GIVING you the same toughness as the creature. Is there an animal requisite? I don't remember off the top of my head, but if there isn't, then I would have to disagree... the spell would then be toughening your skin, and it wouldn't matter what form you were in...
my 2 pawns
Ok, I understand the futility of the Doublet and animal form. But I'm curious about this point.
The spell does not have an Animal requisite and the text speaks of flesh.
Then you should ignore what I said before and let it stack.
My thought along this line are that perhaps large soak totals are more difficult to get then small soak totals. The rules don't back this up, but if I ever thought a bunch of effects were giving a mage an unfair advantage I would feel this could be used as a balancing reason.
There are precidents in the rules, such as stats requiring higher and higher level spells to be increased, so you can't simply cast the same +1 spell over and over to stack bonuses, and that the overall final stat is limited by the magnatude also.
So, maybe increasing soak beyond +20 requires a higher magnitude spell than increasing it up to that point.
In the meta-game, each increase of a point of soak is mathmatically more powerful than the one before, since damage increases for mortal opponents (including Wizards) are limited by stats that require more and more experience to raise.