Still in a very early state, but I released 0.1. Another release will be coming up soon, which introduces a content processor for models to make rigid bodies out of. It isn't very usable at the moment, as it supports Boxes and Spheres only at the moment, but it supports them very lightly. More features will be added on quickly. Link to Sourceforge page : XnaHavok
A video is here:
XnaHavok 0.1 Test
And speaking of which, do those rigid bodies seem like they are reacting oddly?