XP will be around for some time, believe me. I will make the permanent switch to Windows 7 soon, but I will keep a working XP on hand for emergencies. Being something of a classic gamer, I like to dust off my old games every so often.
Windows 7 may become a minimum requirement at some point, but for a very limited number of titles. Look what Blizzard are doing with StarCraft 2: It's DX9 only, and can run flawlessly on 5+ year old systems (with lowered graphics, obviously).
The point is that smart developers will try and make their potential customer base as big as possible, thus cutting XP out of the loop is short-sighted.