Will upgrading to Mountain Lion allow USB Boot?
I have a MacBook from 2007 and it's possible to boot from USB without a problem.

Simply hold down - Option Key / (Alt) on Windows Keyboard on Boot and select where to boot from.

http://support.apple.com/kb/HT1310

If it doesn't work you might have to do a EFI Update

http://support.apple.com/kb/HT3260