Aah, we had this conversation a couple of weeks ago as well. I'm sure it was called something different then though. My, how times are a'changin'! This business sure doesn;t stay still for long does it?
Seriously though. Easy enough.
You need your face on layer tow and your animated mouth (to be shown through) onlayer one.
All you then need to do is mask off the area of themouth on layer 2 which will show through the the mouth in layer 1. The trick in Premiere alone though is to create a feathered mask - which Premiere won;t do naturally. Maybe someone can advise better here but I would create a black/white feather matt in an image editor (Photoshop) and use that in Premiere along with some sort of image-matt effect to punch the whole.
My article/tutorial on fading titles in/out up thepage will explain how it;s all done as it;s the same technique.
Alan
How to: The Conan O'Brien Show Effect 
Linear Mode
