how to install ubuntu 12.0 on MacOS with multi-boot menu
Create an empty partition with Disk Utility under OS X, shrinking the existing OS X partition. It shouldn't destroy data, but make a backup (just in case).
Boot Ubuntu using Live CD or Live USB.
Run the installer, and choose to manually select the partitions. Choose the empty partition as the root mount point (/) and make sure Ubuntu doesn't touch any other partition. The installer should format the partition as ext4 or ext3. Make Ubuntu install the GRUB bootloader (just leave the default settings).
You'll need an EFI boot loader. rEFIt is no longer mantained; rEFInd did a good work with my installation. You can install it from OS X very easily.