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Honestly, that sounds like a horrible idea. First, I'm almost 99.9% sure that you can't charge an iBook through the firewire port. Second, if you did try this it's possible that you could fry something (don't ask me what).
But if you want your iBook to be the guinea pig, GO FOR IT!! Just let us know how things turn out.
But seriously, don't do it dude. Bad idea.
If you really want to charge your computer in the car, get an AC converter. Much safer.
Matt