In the past 3 months I have used 2 Macs at school: first a 12" iBook, and then an 867mhz 15.2" PowerBook. The 12" iBook, from a usability standpoint, is very similar to the 12" PowerBook so I'll give you my thoughts:
The 12" iBook was extremely portable. It was so tiny that in a standard-sized Targus bag I could fit the computer, the power adapter, a Canon G2 digital camera, and a mouse all in the main compartment. It was great for taking notes in class because of the small size. BUT, the damn ports on the side of the unit meant that if you had it plugged in to AC power, and/or used an external mouse, the cables would get in the way of any books/ papers/ etc. you'd have sitting on your desk beside you. This was more of an annoyance than you may think. The 12" screen started to feel a little too small to be comfortable after extended periods of use too, and that made working more tiring than it should have been.
The 15.2" PowerBook is considerably less portable because of its awkward dimensions. That is to say, though the unit is only marginally heavier than the 12" iBook, in that same compartment I mentioned I can now only comfortably fit the computer, and the power box has to ride in another compartment (I don't even carry the camera or mouse anymore), making the bag bulkier. That's a small price to pay though, because the 15.2" screen is so much more of a joy to use than the 12" one. And at least for now the ports are all along the back which makes the unit much more textbook-friendly.
The 17" PowerBook looks to be brutal in terms of portability. Have you seen the launch commercial with "Mini Me" holding it on his lap? Even if you're more than 3 feet tall, you don't want to try to balance that thing on your lap in a crowded lecture theatre. I have the feeling you'd leave it on your desk more often than not, which negates the purpose of a laptop. Further, the massive footprint and side-located ports would be a nightmare to use on a dorm-sized desk if you need to have any books or paper documents with you.
My suggestion? Get the mid-sized PowerBook. It is (or soon will be) as powerful and feature-laden as the 17", but without the extra bulk. Go for the 12" if portability is your biggest concern, especially if you decide to use your iMac.
The 12" would, in that case, be more like a satellite to the iMac. I personally think that because the PowerBooks are powerful enough to stand on their own, they really should, which is why the 15.2" has the best balance of power, screen real-estate, and portability for a student. A 1ghz or faster PowerBook these days is fast enough to be used on its own with no desktop support.