Hello mmoore,

If Maya can't complete a command that it's been given it will advise you to open the script editior for details (shift > s). In the script editor it will list the problems stopping the command and give possible sollutions. In this case there are non-manifold edges stopping the conversion. Maya suggests using polygons/cleanup with the nonmanifold geometry option checked. Maya's methods of cleaning up poly's can sometimes cause more problems than you initially had though, if you make sure that select geometry is set in the drop down menu Maya will point out where the problem lies and leave it up to you to deal with it. In this case the face circled in red is the problem, select and delete it along with the face coloured yellow then snap align and merge the verts in the direction of the red arrow. To finish off, use the append poly tool between the 2 purple edges. Hey presto you can now convert to sub-d, YAY!

Keep at it,

Mat