Comment 5 for bug 597070

I've been suffering this problem on my new iMac 21.5 (11,2 i3, Radeon 4670). All linux live CDs would give black screen even though I could hear the disc loading up.

I've got round the problem for my system by doing two things:
1 connecting an external monitor through the Mini Display port.
2 Using F6 on the grub boot menu to add options - radeon.modeset=0 nomodeset

I don't know if the radeon modesetting works on the external monitor (LG1710, has worked OK on a Mac Mini but will test it on iMac) but it certainly doesn't work on the main monitor.
I've only tried live CDs so far; in 'xrandr' the main monitor is identified as eDP-0 and it lists the full range of supported resolutions up to 1920x1080.