Since my upgrade to XP x64, I've been looking for patches so that my favourite titles can benefit from the extra-spicy new registers, and other cunning tricks.
So far, I've found I have one game that can actually do x64 - Chronicles of Riddick. Which I'll be reinstalling tonight.
There were rumours of a 64-bit patch for Far Cry, that then turned out to be a complete new version ("with more objects" (!?)), but it seems largely rumour (discussion boards suggest that signing up for the patch at the AMD site leaves you with an "it'll be out soon!" type message).
Doom 3 - no x64 patch.
Other games I own... still nothing.
I know it's early days yet, but c'mon! C'mon! If you're a game developer, please compile an x64 version too!