Some Power Macs *can* boot from USB, under specific conditions:
http://hints.macworld.com/article.php?story=20101011105729488
https://help.ubuntu.com/8.04/installation-guide/powerpc/ch05s01.html
However, Apple never officially supported it.
http://hints.macworld.com/article.php?story=20101011105729488
https://help.ubuntu.com/8.04/installation-guide/powerpc/ch05s01.html
However, Apple never officially supported it.