Add Vertex Group selection to Subsurf Modifier

I've run into this issue a number of times where different parts of my mesh require a (significantly) different level of Subsurf.

In these situations I've had to separate the mesh parts into distinct objects, but it would ideally be better for the workflow and object management to keep the mesh parts inside one object.

I realize there may be issues applying the modifier differently to parts of a connected mesh, but it should be totally possible to apply the modifier to separate meshes within the one object.