Ok I've done this numerous times (including yesterday on a MacBook), and this is how you install Vista RC1. You do not need Windows XP:
Partition using bootcamp
Reboot into Windows Vista DVD
Choose the first disk in the installer (200MB EFI) and choose "delete" from the Advanced Options at the bottom.
Reboot the computer
Boot from Windows Vista DVD (a second time, yes I know)
Now setup Vista as normal - it should accept the Boot Camp partition.
"But wait, I have to delete a partition?" you might ask. Don't worry, it isn't actually deleting the partition. Basically, it's just removing it from the partition list, but the partition is still there. It only means Boot Camp Assistant will not run unless you do the following optional step:
Experts only - forget about this unless you know what you're doing
"fdisk -e /dev/rdisk0" in Terminal
Move the existing entries down one slot each (entry 1 goes to 2, 2 to 3)
Edit entry 1 to be of type "EE", start "1" and size "409639"
Write the table and reboot the computer again