Ubuntu on Macbook
The above link covers all the steps necessary to install Ubuntu on a Macbook 13" except:
upon a boot into rEFIT I had to enter its partitioning tool and synchronize the boot table otherwise I got the sad "No Bootable Device found. Insert Disk and press any key" when I would attempt to boot my Ubuntu partition. All you have to do is press the down arrow in the rEFIT boot menu to the Partitioning Tool then just synchronize the MBR.
Ubuntu Documentation
