I spent days getting my Mac Pro to be able to triple boot Mac OS X 10.5, Windows XP (Vista would work fine), and 64-bit Ubuntu. I called Apple no less than 5 times. Here's my experience and how to do it yourself.
I am using an 8-core Mac Pro. I splurged for the extra RAID card. It was in the way, and here's how I finally took it out. If you don't have RAID, you can skip down.
To remove the RAID card, I had to remove much of the inside of the case. DOING IT YOURSELF might violate your warranty. Poster is not liable for anything you do to your Mac. Follow the instructions on how to install a the Mac Pro Raid card here:
Basically, pretend you are installing it, until you get to step 13. Then disconnect the iPass cable and connect it to the original spot on the logic board. Taking all of those pieces out was just to be able to get this plugged back in. Take the RAID card out of the PCI slot, and put it all back together.
Very important!!! After removing the RAID card, you have to reset the parameter RAM to get the drives showing up. Boot while holding down option-command and p and r.
Yes, I know this is crap, that we can't use our $1000 RAID card. If you would like to get it working with RAID, see FakeRaid. Much luck to you, because BootCamp (still!) doesn't support RAID. If you get it working PLEASE respond below.
With the card out and one big 750GB SATA drive in Bay 1, we are ready to install.
Boot off of the OS X disk. Before installing, open Utilities -> Disk Utility and make 3 partitions. Format the 3rd on MS-DOS (FAT32) for windows, and name it BOOTCAMP or something. The second partition we are making is for Ubuntu.
Resume the installation, and install Mac OS X on the first partition. I am using 10.5. After the installation reboot, install rEFIt. It's a boot manager for the Mac, which will allow us to choose the boot drive when we power on the machine or restart. Insert the Windows CD and restart, holding down "c" to boot from the CD.
To install Windows, use the partition it suggests, which should be FAT32 and given the C: drive letter. Format it with NTFS for best results. Not sure what happens if you leave it FAT32.
Important!!! Do not do any partitioning of the drives here. It will hose things and then you can start over.
After installing Windows (rebooting to finish install) insert the Ubuntu Feisty 64-bit Live CD. Download it here.
Install from the CD. When you get to the disk partitioning page of the wizard, choose Manual. It will give you a list of current partitions. You should be able to recognize the BOOTCAMP (msdos) partition, the Mac OSX partition (the first hfs+) and the Ubuntu blank partition in between. Click on it, and remember its disk name for the last page of the wizard. (e.g. sda3). Click Edit Partition, choose 'ext3' for the file system and '/' for the mount point. Yes, format the partition.
Important!!! Don't just click off the end of the wizard. On Step 6, Finalizing the Installation (or something), there is an Advanced button. Make sure to click on it, and install the boot loader on the correct partition (remember from earlier). Finish the wizard, remove the CD after installing, and reboot.
You're done! No manual partitioning, no shell scripts or commands. Apple Disk Utility did all the fun stuff, and now you can boot any of the 3 OSes! Enjoy!
If anyone has additional information about getting this to work with RAID, please follow up.