I could be wrong, but I don't believe that a single, unbroken curve can define the outline of a curve without doubling back on itself.

Next, any surface must be defined by several curves, and not just one. You could chop the curve up into seperate pieces and then create six planes from that, but I fail to see the difference between that solution and a nurbs cube primitive.

I'm guessing that you'd like to create a cube that is covered by a single surface, and not the six stitched together as is the case with the cube primitive.??

If so, then I guess you could just make a nurbs sphere and tweak that into the shape of a cube. It'll just need some playing around to get the cv's into a position that gives you nice edges and nice corners at the same time.

Whatcha making?