Here is the map from the CE promo image. http://cdnstatic.gamesas.com/bethblog.com/wp-content/uploads/2011/08/skyrim_ce_items_x360.jpg
Here is a rough map of the holds I just made. http://i767.photobucket.com/albums/xx316/randomguyface/skyrimholdmap.png?t=1312706872
Now, looking at the CE, where we can see the capitals are written in red and have a large symbol on them, whereas smaller towns have a smaller, less noticeable symbol, we can see the missing capital is Morthal, which I don't think I've heard of in the lore before. It doesn't seem to match up to any location on the arena map. Anyway, there we go, now all we need is to work out the name of the final hold! All I can tell is that it has a fairly long name and seems to start with an H. Has anyone worked out what the name might be?