There is nothing to say that they will definitely do it, but they certainly reserve the right to do so. Given that this is Apple, I'd sway more towards not spending the money until after the ninety days ...
Oh hang on though.
It's not actually a case of spending the money is it? If the app is returned, you are paying out over and above what you made from the deal in the first place.
As I said, it's a tricky one.
I think a better idea would be a trial run. You can try it for ninety days, after that you pay for it, with no refund. The developer doesn't make any money, but at least he doesn't end up bankrupt. I hear that the new API may allow for that.
Apple reserves the right to make returns (and on very rare occasions they will, if a file is damaged on their servers or is corrupted during the download process), but they very clearly do not allow returns for any other reason (including the nebulous
"I didn't like the application" excuse):
App Store Terms and Conditions said:
b. Refund Policy. On occasion, technical problems may delay or prevent delivery of your Product. Your exclusive and sole remedy with respect to Product that is not delivered within a reasonable period will be either
replacement of such Product, or refund of the price paid for such Product, as determined by Apple. Otherwise, no refunds are available.