I seem to remember reading something about distance detection in the Blender Help docs. I’ll go look and see if I can find it again since I’m more or less at a stand still with the course until my new computer gets here (a half hour for a simple render is just too much when trying to keep up). In the mean time, maybe Marc or Micheal will drop in with their knowledge.
Well, since I beat them back… Here’s what I found
Using LMB in the 3D View will place the 3D cursor directly under your mouse pointer.
For accuracy you should use two perpendicular orthogonal 3D Views, i.e. any combination of top Numpad7, front Numpad1 and side Numpad3. That way you can control the positioning along two axes in one view and determine depth in the second view.
To place the 3D Cursor on the surface of geometry, enable Cursor Depth in the User Preferences.
Let me know if that solves it, in the mean time I’ll look for more info…