Huh. Wonder why they're not putting it on XP. Is it just that they don't want to support it, or are they using IE9 as the carrot-on-a-stick that (IMO) DirectX 10 utterly failed at?
I guess it could be because IE9 is probably built upon new APIs that came only in Vista, where Windows switched to HW accelerated windows management (which was pretty fundamental system-level change):