   Havok+ Slim DX
  How complicated would it be to integrate Havok into an engine built with C# and Slim DX?   
  If you are talking about free version of Havok, then you should know that it is distributed only as static C++ libraries. So easiest way I would say would be to create C++/CLR project that wraps operations with physics for your engine. C++/CLR is best suited as wrapper language between managed .NET world and C++ native code.   ;  I agree with the previous post.You can take a look at the source code of  PhysX.Net, hopefully it helps.Someone already started to write a havok wrapper for XNA: hereregards,Kevin   