the only game I know of that has a 64bit version is Crysis, and it comes with both 32 and 64bit.
We may start to see more dual releases in the coming year though.
What will make the difference is somebody writing a game engine that really takes advantage
of 64bit, rather than just a re-compile of code.
Then we'll have a 64bit only game, and we'll just have to wait and see who jumps first.