Mak's might, but 90+% of the P4's can't, nor can the AMD Athlons (socket A), not to mention any laptop CPU not made in the last 18 months or so...
The point basically is that the vast majority of people do not have 64 bit capability, and the vast majority of them will not upgrade between now and the release of 7. Yes, Vista comes in a 64 bit version, but very, very few use it, especially outside of the enthusist community.
Windows 7 will definitely come out in 64 bit, but it would be completely bone-headed of MS to make it exclusively 64 bit unless they want to shoot themselves in the other foot.