I personally prefer fedora more than any of the other distros, but I installed ubuntu on an external a while back. The problem with putting it on the same drive as your OSX system is that the disc utility that comes with mac osx cannot partition drives without reformatting the entire drive. This is pretty much what boot camp does, partitions your harddrive without erasing anything, but it dosn't make the extra partition free space, it sets it up for windows to be installed on the intel macs.
There is some free software out there, one I found on version tracker is called
iPartition. It can partition your drives without erasing the drive. If you are familiar with the console, you can do the whole thing there, but I don't recommend it unless you really know what you are doing.
Edit: Sorry, it looks like thats just a demo. You have to pay to get that, but I'm sure if you look around version tracker you'll find something. Also remember, You cannot partition your startup disk while you are working on it. You will have to boot off of an external harddrive or a CD.
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