Ok, fool proof! (ish)
Install XP SP2 thru Bootcamp. Sort out all your drivers and stuff in XP first, it'll save you problems later.
Now, I can't remember exactly how I did this so..
If I recall, I rebooted, with the Vista DVD in the drive and booted from the DVD.
Go thru the motions, and follow the instructions. At the point where it brings the drive information up, delete the EFI partition. (a 200MB partition which will become 199MB when deleted.)
Now, if you've had the problems I've had, the installation will start but stop during the 'copying files' stage. This is ok. Ideally you want to get out of it after deleting the EFI anyway. (I think you can cancel it actually)
Reboot into XP. Download an app called UltraISO from download.com.
Unpack the Vista ISO into a folder onto the hard drive somewhere.
Once its done, navigate to it, and click on the setup icon.
Choose to Upgrade, and Vista will tell you what programs will not be compatible with it. (Theoretically, if you've just installed XP, you shouldn't have any. If you get any programs that aren't compatible, you have to exit the install and uninstall the conflicting programs)
After this, you should be able to sit back and wait about an hour. The Mac will reboot a number of times, but should finish the installation, and you will have Vista!
Seems long-winded now I've typed it, but it worked and I'm now dual-booting OS X and Vista!
Hope this helps.
[EDIT] You will still have some driver issues (iSight for instance) but the majority of things should work straight away. On my first boot I connected to my wireless network and posted the above topic.[/EDIT]