Why are you putting the word pro in quotation marks? Many people, from professional photographers to professional writers (like myself), prefer the portability of the 12" PB, its faster processor, and the ability to use an external display as more than just a mirror of the desktop. I also prefer the smaller form-factor of the PB to the iBook.
The iBook is nice, but its just not in the same league as the PB. Whether the differences in the computers' features are worth the difference in price is up to the consumer, but since the 12" PB still sells very well, it's hard to believe that Apple will just discontinue the small form-factor professional laptop (which still leaves open the door for a 13" wide-screen PB).
Many real professionals, not just "pros," actually prefer the 12" PB to other PBs and iBooks.
P.S. I know you can work around Apple's limit on the external display, but I do like having the assurance that Apple supports (as in services) my configuration.