I just came by this post by chance. I happen to be working on an Objective-C wrapper for Box2D, the rudimentary stuff is practically done (bodies, fixtures, shapes, collision callbacks). I'll be adding joints soon. The KTPhysicsController and related classes are part of KoboldTouch
and not available separately. Perhaps if there's enough demand I'll look into making a standalone version.
The wrapper's used in combination with the KTPhysicsBodyModel, the model then updates the view's position & rotation, so you can actually connect any kind of view (sprite, label, particle effect, draw primitive, custom drawing node, even tilemaps if you wish) with the physics body. The wrapper also allows you to delete bodies right in the collision callback because it knows when the physics world is "stepping" and if so, simply puts all to be deleted bodies in a queue for deletion right after stepping completes.
There's one other ObjC Box2D wrapper I know of: GBox2D
by Andreas Löw (PhysicsEditor, TexturePacker). Not complete though (no joints) and rarely updated.
I believe the reason why we haven't seen many ObjC wrappers for Box2D is that developers using it on iOS are quite often used to C++, perhaps they even used Box2D before. On the other hand if you wanted an ObjC physics engine then cocos2d developers specifically flocked to SpaceManager and some to Chipmunk Pro, so perhaps those interested in ObjC physics (and didn't care which one was under the wraps) were satisfied with that. I also believe Chipmunk was in greater need for a more readable ObjC interface due to its ambiguous variable names (r, f, m, i instead of restitution, force, mass, and whatever i stands for) - specifically for inexperienced developers. Box2D has a more readable API to begin with, so there's less need to make the API easier to work with, which is one of the driving reasons to start a wrapper besides the language differences.