Why won't my MacBook Pro boot from a Linux Live CD?
Macs use the EFI Firmware (BIOS for Windows).
You need to download a special Mac ISO that allows to boot on both BIOS and EFI Systems.
- For Ubuntu 11.10 you can download the specific Mac image here:
64-bit Mac (AMD64) desktop CD - By experience I know that the following distros allow to boot on EFI systems using the "standard" ISO: Debian, Fedora, Linux Mint
Your questions is related to this here on askubuntu.
You'll probably want to take a look at rEFIt
That's how I booted and installed Linux on my Mac. It's a very nice piece of software.