If you're looking to get a 12" iBook, the differences from what I know of are...
-PB's keyboard is better.
-PB is overall faster. (Faster processor 1.33 to a 1.2 and a faster system bus)
-PB looks better, IMO.
-PB's screen is supposedly better than the iBooks.
-PB is slightly smaller and lighter.
-PB's price of the rev. C cannot be beat.
-iBook's airport reception is better from what I've heard around the camp fire.
-Some prefer the iBook's white look.
-Some prefer the iBook's durable casing compared to the PB's aluminum casing. It all depends on what you like.