I wonder if with the demo version if people will find stuff that is removed from the final version. I personally own 2 copies of a review build of Psychonauts, and there were a few differences from the final build of that game.
I guess we will know for sure when the full game is out and people have played through it. (Or people pirating it now have played through both). I would imagine that the E3 demo is just a sampler and customized so those at E3 would see the game mechanics as quickly as possible...maybe it features weapons you wouldn't normally have at that point etc.