Sorry, booting from USB is not supported in OS X PB.
USB drives themselves won't even show up in the beta, even if you boot from a supported drive. The drivers just haven't been written yet.
By 1.0, none of this should be an issue.
(By the way, this issue has, I believe, already been addressed several times here. Please use the search tool to see if your question has already been answered before going ahead and posting. You'll find out your answers a lot quicker that way, too.)