I believe Boot Camp is only for running Windows on a Mac.
The easiest and safest way to run Ubuntu or Windows (or any Intel-compatible OS) on a Mac is by using WMware Fusion or Parallels Desktop, and running the +guest OS+ as a virtual machine.
I have Ubuntu set up with WMware Fusion, and it is the best running virtual machine OS I have used. I also have Windows XP and Me, SUSE Linux, Solaris, and even Haiku (which is the in-progress revival of BeOS). Most of those are just for fun. You don't need separate partitions, and the guest OS runs alongside Mac OS X.