I've had the game for a long time and really enjoy it playing on XP. So it's rather strange that Microsoft would change the compatibility while nothing's changed in the OS itself. The same version of the OS that went on sale Oct 22 was already available to MSDN subscribers since late August. What's changed in the OS? Nothing really.
The compatibility change on the Microsoft site was a rather recent change (occurred between late Sept and mid October). The status of the game was set by Microsoft as "incompatible" throughout the Beta and RC period of Win 7 development and even until quite recently (before the official release this past October). Understood, but in reality SC4 really isn't "fully compatible".