Just spent the whole day trying. You'll get tons of blue-screens of death, but if you keep trying it will eventually fully install. Took about 12 full tries before the Vista install finally finished an installation. The previous tries ended up with an incomplete install with an error notice that I'd have to start over. Don't be scared off by the BSOD's though, since the final working installation also encountered at least one of those. You may also want to be sure that you use the Alt/Option to choose the Vista partition on the first installation reboot so that it doesn't go back to the DVD startup all over again.
Once installed you have to disable some hardware devices in Device Manager that have exclamation points (unless you have drivers for them) especially the Microsoft Watchdog listing. This will prevent further BlueScreens in Vista.
The key to installing Vista RC-1 on the Mac Pro, especially if you want to install it on a separate drive, is to install XP first through Bootcamp. Once XP is installed, you can startup the Vista installation and do a clean install over the XP. Be sure to take out all your other hard drives (if you have multiple) temporarily in your Mac Pro except the one you're installing Vista on. For some reason the Vista Partion tool during installation doesn't want to install to your designated boot partition unless it's the only bootable drive in the system. Also you should have all USB / Firewire ports temporarily free (except for mouse/keyboard) during the whole installation process.