No, it isn't based on box2d. It's a physics engine that's been built from scratch (although inspired by some code by Randy Gaul).
Box2dweb was originally a C/C++ physics engine, that was ported to flash. The flash version was then automatically translated into javascript. While it works and it can do some great things, its performance is quite poor when compared to ours that was written from the ground up to make use of the strength of the language. However, box2dweb is more complex and lets you do things (such as objects bound to other object with ropes, etc) that you can't (yet) do with wade.physics. But the fact that wade.physics is simpler implies that it's also (much) easier to use.
Wade.physics supports all types of convex shapes, simulates collisions, gravity, air resistance, etc. and does so quite quickly. I think it would be a better choice for most games, even physics-intensive ones (games like angry-birds for examples). Also, it's very well integrated with WADE, so you simply add a behavior to your wade scene objects, and they start being affected by the physics simulation. They also receive 'onCollision' events, in the same way the receive other events in WADE.
