It seems that Joi Ito, Esther Dyson (who I met), Lawrence Lessig, Mena Trott, Cory Doctorow, and Bruce Sterling enjoy putting stickers on their PowerBooks.
I, personally, would never put a sticker on Ambrose (that's my PowerBooks name, and yes damn it I am a geek) mostly because I think that the PowerBook is a darned good looking machine.
However, I did put an Apple sticker on my Thinkpad (which is nameless).