While I love a conspiracy theory as much as the next guy, Adobe supporting WinXP so quickly has nothing to do with Adobe and Microsoft teaming up to conquer Apple. It's very simple. WinXP is just Windows2000 with a prettier (ahem) UI - and since Illustrator has worked on NT/2000 since 1996, there was probably nothing to adjust to make it work with XP.
OS X, on the other hand, was a new ball game for Adobe and their engineers probably had to learn alot of new techniques to get their software carbonized, not to mention that Apple's Carbon libraries were (are) a work in progress, so until they became solidified, Adobe was hesitant to release any software based on it.
As for Adobe, the Register has an interesting article that says all their major apps will be carbonized in the next 6 months. I won't hold my breath, but it would be nice.