I've gotten through most of Under Saarthal, been contacted by the Psijics and followed Tolfdir through the coffins, into the entrance to the dungeon proper. Unfortunately when I approach the door, there is no prompt to open it. The door is completely inert.
I've tried loading a save from before I entered the dungeon, and run into the same problem. I wonder if anybody knows a console command to remind the door that it's a door?
Alternatively is there another entrance to lower Saarthal? I'd give up on it, but you really need to finish this quest in order to go any further into the College of Mages.