I don't see Apple offering a 13.3" MBP, because as others have said, it'll overlap too much with the MB. I also wouldn't support a 12" form factor, because you are sacrificing too much (in my opinion) to get it that small, like a decent dedicated graphics card, expansion ports, expansion cards, etc.
What I do think is an option though is to introduce a 14.1" MBP. I believe that's slowly moving to be the preferred form factor since 15.4" is just slightly too big to be conveniently portable. And now, you can pack just as good components (namely a decent mainstream GPU like the X1600) in a 14.1" form factor as you can a 15.4" so it's certainly a great option. In fact, I might even suggest that Apple replace the 15.4" MBP with a 14.1" model to avoid having too many products. In terms of features you really wouldn't sacrifice much if anything, and I'm sure Apple can get the weight down under 5lbs, which is quite portable. The 17" MBP can remain as the full featured, luggable model, and the 13.3" MB will remain secure as the smallest, affordable model.