I have no problems at all with them not supporting XP. It's 10 years old, and if they've decided to make use of features of Vista and 7 not available in XP, then good for them. Seriously.
I'm also inclined to think that any non-enterprise user still running XP and IE isn't going to manually upgrade to IE9 without being forced to. Those that would are also able to install any of the alternatives that do run on XP