I'm not really in a position to comment yet, as my iBook isn't arriving until the 9th September... (I thought it was coming yesterday T_T )
Anyway, I was in a *very* similar predicament to yourself - the laptop wouldn't be my main machine (I have a desktop, Windows, that I have to keep using for a lot of my work as that is what my department runs - computer science). Anyway, the laptop would mainly be used for browsing etc, with the programming if I can.
I saved myself a lot of money and went for an iBook. The 12" model too - I figured that since my eyes are pretty good, there was no need for the extra screen size when you get exactly the same resolution - for programming, it's all about real-estate.
Obviously, I would have preferred the PB, but for the money and what I'll be using it for (after all, I'm still tied into my desktop for another two years), I couldn't justify the extra for what will basically be a suped-up web browser...
So that's my two cents.