this depends on a number of things.
if you will be using it to do paperwork, then the powerbook's keyboard is sturdier than the rest, if you don't mind using a touchpad or paying extra for a mouse. also the 14 inch ibook is too fat and heavy for the 14 inch category and the LCD experiences problems when you put pressure on the cover, meaning it's not a good build (good deal though). the 15 inch is just right as a laptop: good speakers, good screen, good build and good size. if you are willing to wait till January Apple will realease the new ibook that will have Intel's pentium M processorsin it, or you could wait till they do the same with the powerbooks. you might also want to look in on the possibility of getting an education discount.
also, i'm not familiar with their battery life since my experience with apple laptops has been limited.
for the 17 inch i mac, it is the best computer of the three but you would have to accept the limitations that come with a desktop. I also think apple will do a intel upgrade on this computer in the early part of next year, but the G5 is a good processor and might not be as big an upgrade as it would be for the laptops. also, with an imac you are not getting the same type of upgrade capabilities that exist on other desktops, including the powermac.
the imac seems like the best of the bunch.
random question: UPS left a note in my mail box asking me to pick up something tomorrow, what will it be? (i also don't know)
DOPE, someone posted while i was writing that. so here i go. the Xbox 360, just because me and sony don't get along right now.