I was pleasantly surprised that changing a Mac Mini to dual boot Mac OS X and Linux was relatively painless. I more or less followed the instructions here: http://www.rickycampbell.com/ubuntu-easy-install/
I ran the Boot Camp thing in the Utilities folder and allocated the disk half and half. (I was kind of annoyed that it assumed I was going to install Windows.)
Then I installed the rEFIt boot manager: http://refit.sourceforge.net/
Finally, I installed Ubuntu 9.04. The only problem now is that the display comes up in 800 x 600 when I boot into Linux.