I just managed to get Mac OS X Tiger, Windows XP, and Ubuntu 7.10 all installed on my Macbook. Everything runs smoothly except for one thing, I think something in the Ubuntu startup is putting the gpt and mbr out of sync. I thought it was something to do with fsck so I turned off checking for each partition in /etc/fstab but that didn't fix it. Ubuntu seems to be running fine, I just need to make sure to run gptsync in Ubuntu and in the rEFIt terminal... but that's kind of a hassle. Anyone know a good fix for this? Here's some system information:

Macbook Core 2 duo
partition 1: EFI
Partition 2: Mac OS X
Partition 3: Ubuntu (boots using lilo)
Partition 4: Windows XP

After looking through the refit troubleshooting page, I fixed it. Turns out the issue was that the Ubuntu partition was flagged bootable. I just turned the bootable flag off for the Ubuntu partition and it has been working great since then.