Ihar Kalasouski has just finished a complete port of Erin Catto's Box2D C++ phsyics engine to the .Net framework. He is calling it Box2DX.
As a bit of a history lesson, the Farseer Physics Engine is based on Erin's 1st version of Box2D, Box2D Light. Farseer is a pretty heavily modified version of the engine,but that is where its roots lie.
The new version of Box2D that Erin released many months ago is a complete re-write of the original Box2D Light engine. It is full of useful features, has full docs, and is very nicely architected.
Box2DX, Ihar's c# port mentioned above, is a full 1-1 port of the NEW Box2D engine. Ihar has purposefully mapped all methods and functionality 1-1 into the c# version. This is nice because Box2D has some very nice documentation that now applies directly to Box2DX as well. Ihar has also taken the pains to add code comments to all his methods. (Unlike the developer of the Farseer engine...me!)
I did a compile test with Box2DX to see if it would compile against the Silverlight framework. It did with only 1 exception. Box2DX uses the ICloneable interface in 1 place and that threw an exception. As far as I can tell, that interface isn't really necassary and removing the reference to it is all that was needed to make the engine compile. I'll see if Ihar is willing to remove it from his source so that no modifications are necassary for the Silverlight compile.
I haven't done anything with Box2DX other than compile it under Silverlight and run the included "HelloWorld" test.
Once I get the alpha version of my Silverlight game, Diver, out the digital door, I will probably take some time to do a few quick examples with Box2X in Silverlight and see how it works.
Links:
Box2D: http://www.box2d.org/
Box2DX: http://code.google.com/p/box2dx/
-Jeff Weber