Check this new behavior recently published with full source code at the Microsoft Expression Blend Behavior's gallery. It allows us to bind the mouse movement to a plane projection of a FrameworkElement without any line of code.
Get it here http://gallery.expression.microsoft.com/en-us/Project3DToMouse and learn a bit more about it here: http://silverlightguy.com/2010/06/03/project3dtomouse-a-mouse-powered-3d-projection-behavior/