Some models of the Powerbook have Radeon 9700 graphics cards, while others have GeForce FX Go 5200s, while the iBooks have Radeon 9200s. The Powerbooks have faster processors. The Powerbooks go up to larger screen sizes. The Powerbook also ships with a default 512 MB of RAM, while the iBook ships with a default of 256MB. The iBook seems to weigh slightly more.
The Powerbook has a metallic case, while the iBook has a plastic one. Other than that, I don't remember any really big visual differences between them.
Those are all of the differences I've gathered from their PR pages.