In playing around with the new SpriteKit in iOS 7, I got this assertion error:
Assertion failed: (def->bodyA != def->bodyB), function b2Joint, file /SourceCache/PhysicsKit/PhysicsKit-4.6/PhysicsKit/Box2D/Dynamics/Joints/b2Joint.cpp, line 173.
I guess Apple licensed the Box2d library or hopefully at least contributed to its development. I haven't seen any definitive mention of this connection but I may have missed it somewhere.
I wonder how it differs from the actual Box2d version, other than apparently not including all of the joints.