T-Splines, PowerSubdNurbs, and other "converters" that have been referenced here are quite limited in the number of polys they can process into a nurbs surface. These tools are intended to be able to manipulate a nurbs surface based on a polygonal "reference" surface. The "conversion" is a by product.

If anyone has found something that can handle millions of polys, please indicate what it is. In my experience, the only applications that can fit a nurbs surface to millions of polys are Geomagic and RapidForm.

I am particularly interested in Resurf3D with their RhinoResurf application, which looks like it can handle more polys. Unfortunately I have not been able to test it because the Rhino trial and the RhinoResurf trial don't play well together.