I have been running Vista Business 64 bit on my ThinkPad T60 since it was available last year. There were some teething problems at the beginning, but SP1 addressed most of the problems. I have not had any issue post-SP1. (knock on wood)

Presently, the most compelling reason to use 64 bit is to use 4 GB RAM or more. It is now very easy to exceed that number with the cheap RAM deals around. When upgrade time comes around, 32-bit OS will need to be reinstalled with 64-bit equivalent, but 64-bit OS will only need to be rebooted. To prevent the hassle down the road, get Vista 64 unless you have legacy applications or equipments that don't work in Vista 64.

I take the same approach wrt XP. Are there legacy applications and equipments that only work with XP? The XP compatibility layer in Vista is pretty decent. Most legacy applications that don't play with Vista have conflict with the new security model. These programs expect to run with full administrator privileges, but Vista does not give full privileges unless explicitly to do so via UAC approval. Usually right click on the program and select "Run as administrator" does the trick.