I don't really know that much about nurbs, but I'm think a curve has to be planar to make a planar surface... Hence the name

As I said I don't know much about nurbs, so I'd probably advise you to convert to polys, add faces to close the hole and then convert to sub-d's or smooth the polys... I can't remember the convert settings, but Kurt goes over the process in the free cartoon dog tut...

-Blomkaal