I, too, think iBooks are nice, but the 12 PB just has too many pluses. In addition to those listed, it can have a superdrive, has a much better keyboard IMHO, and has a better build, fit, and finish. I expect it will last much longer.
In addition, the 12PB can use an airport extreme card for better wifi with an airport extreme base station, and I believe it has better video for use with external monitors.
I think the screen is also better, but that may only be a matter of opinion.