One thing I need to do right now is convert from our camera input space (-1 to 1) to 3D space. The current prototype is using a 3D perspective world, so converting the 2D cursor to a point on a plane in 3D space will require the use of the ViewPort.Unproject() method, which is apparently broken in some cases.

This thread on the XNA forums provides a replacement function, which I'm going to use.