Alternatively you could just run it under VirtualBox
(http://www.virtualbox.org/) or your preferred virtualization
solution.

Ubuntu appears to have some docs on installing Ubuntu on a MacBook,
may or may not be helpful:
https://help.ubuntu.com/community/MacBook

rEFIt appears to have a solution: http://refit.sourceforge.net/
http://www.slax.org/

Also, there is always the wise-guy option:
http://lmgtfy.com/?q=mac+linux+usb+boot ;)

-- 
Andrew S. Zbikowski | http://andy.zibnet.us
IT Outhouse Blog Thing | http://www.itouthouse.com