For general installation information see iMacIntel
iMac12,2 with latest linux kernel 3.11 : almost full support.
- efi boot all right since debian wheezy
boot with radeon.dpm=1 to get dynamic power management (kernel >= 3.11 with firmware-linux-nonfree >= 0.40)
- brightness control is in /sys/class/backlight/radeon_bl0 (maybe on earlier kernels too)
iSight needs the firmware from osx to be installed as explained on iMacIntel.
- speakers, jack detection, mic all work
- untested: thunderlight port
- suspend, hibernate work