Not all Macs can install Vista 64; e.g. only post-2008 MacPros can; pre-2008 can not apparently. There is an explanation on Apple's website. It is something to do with a 64 bit EFI.

I beg to differ. My Mac is a pre-2008 Mac Pro and it would be most offended to know it is not supposed to be running Vista 64. :-)

The 64 bit version of boot camp translates the Macs EFI to a bios simulation for Vista (or XP)

How did you get it to work? I have Vista Ultimate Retail SP1 (with both 32 and 64 bit versions). When I try to install the 64 bit version it stalls when rebooting. I checked the apple website and it states that only certain models are able to run Vista 64. See http://support.apple.com/kb/HT1846
I would love to know how you got it to work. Did you use bootcamp assistant? Are you using Vista pre sp1?

Hmm, I had no idea I had done something unusual. :-)

I originally installed Vista64 Ultimate using a pre-release copy and the beta of boot camp. Since then I have upgraded to SP1 and the official 64bit boot camp drivers.

I seem to recall that if you try to boot from a copy of Vista64 SP1 you hit a screen asking you to choose install type which requires input from the keyboard, but no keyboard is active. So maybe it has to be a pre-SP1 version.

Also I took advice from various sites such as:
http://www.wintellect.com/CS/blogs/jrobbins/archive/2007/02/15/vista-x64-on-a-mac-pro-totally-awesome-dude.aspx

I think it was important to install Vista64 on a seperate disc, and pull the Mac disc during the Vista setup. I downloaded drivers for the Motherboard from Intels site (it's a 5000 series workstation board)
and audio drivers from realtek. Initially I had no wireless network or bluetooth devices working but that was fixed with Version 2 of boot camp.

Hope this helps, let me know if you need any more info.