"...The only thing that really comes to mind is the sensable system: http://www.sensable.com/ although I don't think it pushes NURBS surfaces around directly, it's something like pushing around voxels and then fitting NURBS to it at the end or something along those lines...."

We have a few Sensable devices at the office. There's no importing of or using Nurbs for sculpting, that's purely voxels as u mentioned. The software does have a few Nurb-ishy surfacing commands similar to network, loft, etc. But the end result is always voxels, and ultimately exported as a mesh...

Regarding models converted from subd to nurbs - the topology of the converted models is often not ideal to work with downstream as the individual sub-surfaces are always untrimmed rectangular surfaces. They have a tendency to get pinched near the corners and this can make it difficult to add local features without first reworking those regions if your new feature crosses those pinched regions. Not sure how IW's converted results are constructed but I imagine it's similar to Tsplines in nature. Although they claim at least C2 along all edges, while Tsplines corners were only C1. I'd be interested to see IW's output and see if it fairs better (look ma, I made a funny!)...

jonah