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Use disk utility to partition your hard drive so you have at least 20GB of free space (so partition your harddrive into two parts, then delete the part that ISN'T your Mac partition)

Then, download and install REFit on your Mac.

Have Ubuntu on a DVD or CD, then put it into the Mac, reboot it and hold either C or Option to boot from disk (cant remember which,
google it) then follow the Ubuntu instructions, and make a new partition for Ubuntu and a linux swap partition out of the free space you made (DONT touch the Mac partition as that's where OSX and your Mac files are)
Ubuntu should make a compatible partition out of the free space and install on there.
When you finish and reboot your computer, REFit will open before anything else and give you the option to either boot into Mac or Ubuntu, or I think you could also just hold option to pick which partition to boot from.

Hope this is somewhat cohesive and helpful.

EDIT: believe you need a disk with your drivers on it as well, if you have a lion install disk then use that, if not then use boot camp to make a driver disk or figure out another way. Since Mac and Linux are both unix-like it shouldn't be too hard. You put the driver disk in after installing and booting to Ubuntu.
 
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