Nick,

I took a look at the geometry and found the main problem to be a duplicate edge (see the image below). To fix it I just deleted the polygon, selected an edge, then picked fill hole to replace the poly.

You also have a pair of unmerged verts (see image)

To see border edges select display>polygon and turn on show border edges.

Minor nits:

The geometry at the tip of the upper and lower jaw is a mess. right now you have a bunch of verts all merged to a single point. I'd consider cleaning those areas up a bit - I think Kurt cleans those up in that tutorial at some point.

Also you have a lot of n-gons (for example coming out of the eye corners). Once again I think Kurt cleans these up later in the tutorial.

So fixing one relatively innocuous laminar edge and the model converts to subd.

Other then that you appear to be progressing nicely with the tutorial. I am sure Kurt would be pleased. Go Go Go!