Its a little more complicated then you let on.
Bootcamp won't recognize the disc as a Windows Installation CD so you'll have to start the install manually. It should work though.
Download bootcamp.
Run the installer.
Make the partition.
Close bootcamp.
Insert Ubuntu CD.
Restart (holding down option after the bong)
The mini should start up from the live CD.
You can install, but make sure you install on the correct drive.
Is possible that something might not work or go horribly wrong, so back up all your files before you go messing around. Beta software, unsupported operating systems, and altering of partitions generally shouldn't be mentioned in any one situation.