I just installed Ubuntu on my Mac under Parallels using these instructions:
How to install Ubuntu 7.04 in OS X using Parallels Desktop 3.0
It seemed to go smoothly. Of course, Ubuntu immediately wanted to download a pile of updates, but that worked ok.
The only problem is that you seem to have to manually turn on the networking within Ubuntu every time you boot it up. The instructions mentioned this, but I thought it was just during installation. There may be a fix, I haven't looked yet. I don't think it's a big deal because normally I just suspend the virtual machines rather than shutdown and restart them. Hmmm... that's assuming the network stays turned on after being suspended, I haven't actually tested that.
Parallels has a pre-installed Ubuntu virtual machine, but the downloads were very slow so I gave up. But I should be able to take the virtual machine I've now created and copy it from my Mac Mini to my Mac Book.
Aside from curiosity, one of my motivations for installing Ubuntu is to get back to working on porting Suneido to Linux. I should be able to do a lot of that natively under OS X but I assume I'll still want to work under Linux as well.