This is how you do it... BTW it is explained in your Powerbook's User Manual. You need a 6-pin to 6-pin Firewire cable. Also you can use either the 'book or the PM/iM/eM as a Firewire disk.... the setup is similar.
1) With the Powerbook/iBook Off, connect it to the desired machine.
2) Powerup the notebook and hold down the "T" key. The startup screen should change into a Firewire Icon.
3) Your 'Book's HD should show up on the machine you want.
4) When finished simply drag the HD to the Trash... Then the book will continue to startup normaly.
Hope this helps...
TEG