Unless MS all of sudden really makes Seven really power-hungry, comparable to Vista. They need to stick with x86 for their novice user who does the install themselves, who doesn't know what the hell a x64 processor is or has an "older" computer with an x86 processor. Also, if I remember correctly, Seven can run on a 1ghz processor, and there are x86 processers running at 3ghz+. And don't forget it, MS is a businesses aimed at making money for their stockholders, so to expand their market the must still use x86 in Windows 7.
The next release of Windows (after Seven), will be a radical change. Either in the cloud or something like the jump from XP to Vista (with all the power-hunger). Probably, this is the final hoorah for x86 simply because of it's speed limitations.