My job involves doing technical support for Rhino.
A common question I get is: "I have a scanned part and I need to turn it into a NURBS model". This can be a difficult and time consuming task but it can be done using native Rhino tools or a reverse engineering plug-in for Rhino
A little over a year ago, I wrote a document that describes this process and the techniques involved using just native Rhino commands. It covers analyzing the mesh to determine what kind of clean-up may needed. Once the mesh is cleaned up and made water-tight, it is ready for rapid prototype printing.
The second part of the document describes how to manually reverse engineer NURBS surfaces from the mesh.