Hi,

Initially when I saw the image I thought that you simply hadn't merged the vertices, but since you mentioned that you had tried that, it got me interested.

After a failed attempt at some more vertex merging around the mouth area, I decided to simply rebuild the entire area, while systematically testing to see if the problem was gone. Doing this I managed to narrow down, what I think may have been, the origin -- half-way accross the upper lip. I deleted all of the faces that had any shared sides or vertices to the problem area and built them over again. I also did the same for the lover jaw.

Additionally, I looked over the rest of the model and noticed that on the back of the head were two more unmerged vertices, which might otherwise have caused you more trouble in the future. I took the liberty of fixing 'em.

By the way, there's an easy trick to quickly tell if there are multiple vertices where there ought to be only one. Marquee select the area that you think has the problem, and then look at the very top part of your screen, where it says: Maya.... etc. There, after the directory path of the current file, you will notice something to the affect of "polyToSubd1.vtx[###]" The part with the number in it refers to the vertex number(s). If you see this followed by three dots or if there are multiple numbers, then you have selected multiple vertices. Obviously, this cannot be the case if you think that you only selected one...

Hope that wasn't too confusing :p