Unfortunately, I think it's the age of your iBook that's making the difference here. Coming from someone who owned BOTH the new MBP and the new MB, the performance difference is not substantial.
In fact, if you upgrade to the 4GB of RAM on the high-end MB, it's pretty much identical to the entry MBP, save for the slightly improved graphics card and faster HD. Personally, the only (slight) difference in performance that I noticed was with startup/shutdown (2 sec-ish), but I attribute this to a slower rpm HD. I'm going to wait until SSD prices drop and then toss one in.
Moral of the story - The word 'pro' is relative, depending on what you use the computer for. In my case, the high end MB is definitely a pro
Oh, and I use the MB for essays, internet, movies, and intense photo editing. It passes all of my tests with flying colours.