How do I make my 1.5 GHz PowerBook G4 boot from a USB stick?
I have done this many times since I posted this question. 


Create usb stick like this:

sudo dd if=/&lt;path to iso file&gt;/&lt;name of iso file&gt; of=/&lt;path to usb stick&gt; bs=32768 conv=notrunc,noerror,sync

Put the USB stick into your Mac and press the power button while holding down Command ⌘+Option+O+F
At the prompt, type the following:

boot usb1/disk@1:,\\yaboot



Note: If you are booting linux then use ,\\yaboot, but if you are booting a Mac OS use ,\\tbxi.

Note 2: If your stick is in the right side of the machine, then you need to do this:

boot usb0/disk@1:,\\yaboot


Note 3: If these commands don't work, then there is most likely a problem with your media or iso file.

I have found that Ubuntu, Debian, and openSUSE work great. I cannot get Fedora or Gentoo to boot this way yet. My optical drive does not work anymore, so this is how I install OS X on my machine. I made a live USB stick of the OS X install disc this way.