I just switched my MacBook over to Ubuntu
I made three image backups of the hard disk on my MacBook (yeah, I am careful like that) and just used BootCamp to set up a small OS X partition and an Ubuntu 9.04 partition. This worked for me, but I make no guarantees for you (<grin>):
Boot with OS X install DVD, wipe the disk, and installed to the entire disk
Without doing anything else, I ran BootCamp and grabbed about half the disk for "Windows"
I inserted a Ubuntu install CDR, and chose "advanced" partitioning. I deleted the fat partition on /dev/sd3 and made it a bootable ext3 partition with "/" as the mount point
When I boot my Mac, I hold down the option key to switch between OS X and "Windows" (ha!).
This is really non-optimal, but I had a false start: the first time I ran the Ubuntu installer, it recognized OS X and offered an express install. Anyway, I ended up with a small 2 gig partition /dev/sda4 that BootCamp boots the first Ubuntu install. I simply edited the /boot/grub/menu.lst file to set /dev/sd3 as the boot disk.
Now I can "option key choose my OS
Now that both operating systems work fine, I took the time to install all security updates for both operating systems
I would have liked to go through these steps again, leaving out the first false-step Ubuntu install, but I need to install my dev tools on Ubuntu and get back to work.