Bethesda Softworks is going to be there, so they could be talking about:
-Fallout: New Vegas
-Brink
-The Natal game
-The TES MMO
-TESV
Obsidian isn't going to be there so they're probably just there to show off Brink and New Vegas.
My guess is Brink, maybe with some brief mention of FO:NV... Their twitter feed seems to talk about Brink quite alot so it's their current Golden Child if it were.
As for project Natal, I'll believe that when I see some sort of demonstration that doesn't involve scripted actors presenting a "what if" situation. Maybe I'm just jaded from seeing failed attempts on a similar line, like Playstation's Eyetoy, Sega's Activator, and other horrible failures. Such a game would also not be possible for the PC, so would essentially be one big giant "F U" to PC gamers and Modders that would certainly not end well. If it were EA we were talking about, I could see it happening, but gamesas has shown a bit more intelligence in this sort of thing, so it's highly unlikely.