PB+wireless networking+warm spring day=Awesome
When I lived on campus, I would put my ABS in the window and go outside and lie in the grass, surfing the web. It was even better before everyone and his brother had WiFi cards, people were in awe.
What everyone has said about laptops and portability is right on. Being able to pick up my computer and take it to the library, the coffee shop, class, on the train, in the car. It's wonderful. Besides the convenience of being able to take it anywhere to write papers and such, I've spent more than a few long train/car rides occupying myself by watching movies on my computer. I will probably get a desktop of some variety after I graduate (less than 2 months now!), but I will hold on to my PB (which will be 4 years old by then) because the option of portability is wonderful. Trust me, you will be glad you got a laptop. I can't really speak to PB/iB, except to say that both are very capable computers. I'd say, if you have the money, go for the PB, if you're worried about spending that much, go for the iBook. But either way, get a laptop. You'll be glad you did.