I need to install Ubuntu on a Macmini 6,1 machine (late 2012) and as it was not without pain, I think it’s good to share my experience…
The first steps to create a partition and a boot system for Ubuntu were quite easy.
Here is a quick sumup :

• boot on Mac OS
• download rEFIt http://http://refit.sourceforge.net/ and install it (you have to manually install it because non-verified package installer could not start)
• resize the Mac OS X partition with the Disk Utility
• boot on a Ubuntu 12.10+mac 64 bit DVD (thanks to rEFIt) and install it on the new partition

At this step, the system is not working fine. There is no video acceleration, display is not recognized, no network driver…

The first thing I fixed is the network, so I can do update more easily.
I used my Samsung Galaxy SII to share a WIFI through USB and get the Macmini connected.
To make wireless working I followed the procedure explained here : https://help.ubuntu.com/community/Macmini5-1/Precise#Wireless_.28AirPort.29.
And to fix ethernet, I followed this : http://ubuntuforums.org/showthread.php?t=2078320 and made change like in post #5 then reboot.

To fix video I ran :
sudo add-apt-repository ppa:glasen/intel-driver
sudo apt-get update
sudo apt-get install xserver-xorg-video-intel intel-microcode mesa-utils
The intel-microcode may be useless.
The Intel HD4000 is now working but my full HD display attached to thunderbold with a miniDP to VGA adaptor is still in 1024×768.

To fix the last point I followed this : http://shanereustle.com/blog/force-screen-resolutions-on-ubuntu.html. This method is more convienent than manually modifying xorg.conf but this is not persistent so i made a small script that run thoses 2 commands on startup.