I have a 1.5GHz G4 Powerbook. Until Saturday, I'd just been running the latest Mac OS X Tiger release on it. It's got an 80GB internal disk. I decided that I wanted to make it a dual boot system.
Read it here
From talking with someone else who'd already done this, I was under the impression that I was going to have to backup all my Mac files, divide up the disk with the Mac OS X Disk Utility, then restore the files and then load Ubuntu into another partition.
From googling around, I found out that you don't have to do that. You can just resize your existing partition. You can also do it for free. In other words, you don't have to shell out dollars for special software to resize partitions.