It would seem to me that you only have 1/2 a model from the freebie tutorial Kurt provided.

As far as I can remember, you need to duplicate, combine and then merge the verts. Once thats done delete the history and then attempt the conversion.

If its not that and I am talking ****, then reset the settings in the poly to subd options and ensure there are no non manifold faces on the model by running the clean up tool.