I have been trying to get my iMac mid 2011 to dual boot Ubuntu with Lion or Mountain Lion. First I tried with an ubuntu DVD with no success then a formatted USB drive using the instructions here:
Using the USB and these instructions have worked a treat for me on both a MacBook Pro 2012 and a Mac Mini mid 2011 and I have dual boot working great on both. All I need to do is press Alt on startup to choose what OS to boot. I did not need to use reFit or reFind (as they did not work for me). http://askubuntu.com/questions/219886/impossible-to-boot-ubuntu-on-a-usb-stick-os-x-montain-lion
On the iMac I have tried in turn a) reFind, b) pressing Alt on boot, c) press C on boot up and none has been able to identify the USB (or CD) boot disk to boot from it.
Has anyone tried this on an iMac and can they help me out. Many thanks in advance