anything that has a Uninorth chipset can boot from USB, this includes Sawtooths (Sawteeth?) and the first iBook G3's
on these early systems you can even USB boot simply by holding down the alt key, and it will even work via USB cards as well
anything with a Grackle/MPC106 chipset cannot boot from USB simply as their firmware does not include a mass storage driver, theoretically someone could write an OpenFirmware mass storage driver for these systems and load it in as a NVRAMRC patch or such if space allows and boot from USB