I have a Mac mini (Mid 2011) - and while the live cd works great - once ubuntu is installed, it will either freeze during bootup - or as soon as the gui is ALMOST all loaded up. (the icons appear on the left, however the top bar is blank).
I am using Refit, and tried installing both the 32bit and 64bit version. I setup a / partition as well as a 6GB swap.
I have been able to pinpoint this issue with the fact that I have a second monitor plugged in via the HDMI port.