I found detailed instructions of how to install Vista on a Mac Pro!!!!111!!
1. Insert Vista DVD
2. Turn on computer
3. Hold ALT while computer is turning on to give you the boot menu
4. Select the pretty DVD icon
5. Install Vista
6. Enjoy Vista on your Mac Pro
(BTW i actually just made those up. However as i said, i've seen Vista RC1 install onto a mac pro, i don't see why RTM wouldn't.)