I have a MacBook 6,1 with both, OS X and Windows XP partitions. I try to boot from my USB drive which has Ubuntu installed and working (tried on friends computer) and it booted fine.
I try to boot it and it boots into Windows. I am using "rEFIt" and "Live-CD's" work. and USB's and external hard-drives don't seem to.
I tried this guide, and the problem with the answer is, that I am not running a "Live Boot" but an actual install. I took a Live CD and installed Ubuntu onto my flash-drive (32 gig) and I use it as an external hard-drive.