I've just worked out how to get my iMacs to boot into linux by default. Previously I've blogged about my efforts to install Debian on an iMac
and a macbook air
. The arch linux people have documented
how to bless you're linux boot loader. They say you need to boot into Mac OSX, and then mount the efi volume. Over on themacadmin.com they describe
how to mount the efi volume.