The Orbis Sanctum bunch have found a way to get out of Valhalla alive. By using a small hole in the wall of the level, one can simply crouch and walk straight out. You can find a video documenting the discovery in the Valhalla sub-forum.

Well done guys, let's hope Bungie fix this before the game's release on September 25th!

UPDATE: The guys from DefyLogic have used the above technique and a bit of grenade jumping to get even further out along the edge of the water. Death barriers may squelch any attempts at real exploration, but still, this is cool stuff. :)