I have an iMac configured as follows -
Screen 27" CPU 3.4 GHz i7 Graphics AMD Radeon HD 6970 1024 MB
I downloaded Ubuntu version 11.10 64 bit ISO and burnt that to both DVD and USB stick as per the instructions on Ubuntu's download page.
Neither will boot. Symptoms are as follows -
DVD: When the iMac is restarted and booted from DVD (labelled Windows which isn't mentioned in Ubuntu's website instructions) one line is displayed against a black screen displaying a message about the developer and date. After 5 minutes the message hangs and the DVD ceases to spin.
USB Stick: Strangely I have to select the EFI boot CD icon which appears after holding down the Alt key. A text menu appears offering me to try Ubuntu without installing. I select this and the screen goes blank and stays blank.
Lastly, after writing Ubuntu to DVD and USB stick, neither could be read by OSX making the instructions to eject them as per Ubuntu website's instructions useless. This might help?