I had previously setup rEFIt on my Mac mini and set it up to dual-boot Mac OS X and Debian.
Recently a dist-upgrade broke X11 beyond repair and since I was helping my cousin switch to Ubuntu from Windows XP, I decided to try installing Ubuntu on the Mac mini.
In short, it was incredibly simple and straightforward, and I even ended up with grub on it!
I have another Mac mini which runs Mac OS X only at the moment and I'm definitely going to install Ubuntu on it as well - maybe even with Xen!