http://www.macworld.com/weblogs/editors/2005/06/usbharddrives/index.php
http://docs.info.apple.com/article.html?artnum=106474
Seems that if you have a copy of super duper you can create an image that you are able to boot from USB hard drive. It just doesn't work other wise it seems. If you noticed the apple link that USB drives aren't even supported for PPC platform either (Documentation creation was Sept. 2001).