I'm looking for a 2D physics engine for Windows Phone 7.
Is there a port of something existing for use?
I've worked with Box2D before on a C++ project and found it pretty simple to use. There's a port here for XNA and the release notes says it's been confirmed to work on Windows Phone. But I can see from Farseer's website that it seems to have more features than Box2D so just take a pick.
I might as well mention the only significant problem I had while working with Box2D (mostly due to ignorance and being the first physics engine I used) which took me a while to figure out: Box2D is tuned for a MKS (meter-kilogram-second) system. Thus it expects your game objects to be reasonably sized, somewhere between 0.1 and 10 meters according to the documentation. I was unaware of this and created all my physics objects in pixels. The result was... sluggish... as expected of trying to move tanks with the size of a couple football fields around. Other than that everything worked great.