There are two known problems with iTunes on x64. Since I paved my home desktop box to Vista x64 this morning, I knew I was going to need to figure out how to get iTunes running: not only does it stream music for me (from my home theater PC in the living room), but it's also my iPhone sync source.
Before you install iTunes, make sure 32-bit VBScript is enabled.
After you install iTunes, you'll need to install the 64-bit Gear drivers. iTunes uses Gear its ripping and burning support, but they only ship x86 drivers in the box; when you launch iTunes on x64 it yells at you about missing services. You can install the x64 Gear drivers from the Gear web site.
I couldn't find both of these pieces of advice in one spot, so I figured I'd save it here, for myself if nobody else. :)
Update: Apparently, iTunes isn't quite fully functional; in 64-bit form, you can't sync an iPhone. This hasn't affected me yet because my music is on my Mac mini, and for contacts and calendar, I sync to a 32-bit VMware VM on my MacBook Pro (let's not even talk about how totally broken the iPhone info-sync appears to be with Entourage).