One reason I say go for a Powerbook over an iBook is the ability to hook up an external monitor and use it at a high resolution, something the iBooks can't do. While an iBook does feature an external monitor port, it forces you to use the resolution of the laptop on it, and the iBooks limit is 1024x768, probably not a good resolution for serious Photoshop work.

Excellent point and you may want to rule out the 12 inch powerbook for a similar reason. Their max res is 1600 x 1200, but with a VGA not DVI connector so you can't use one of their sweet displays if you want a really big workspace. But the 12 is a lot more portable so if you want to be able to take it to class with you it may be a better choice. It comes down to how much cash you want to spend and how your are going to use the laptop.

-Mike