how do I make a 3rd person over the shoulder mouse look?
In the resource area at Unity3d.com, there is an example project that you can download that is a 3rd person shooter with this exact setup.
You can look and see how they used the camera scripts.
When I re upload my source, it makes it very easy to make one, keep an eye out for
http://forum.unity3d.com/viewtopic.php?t=62129