E3, while not the glamour/lights/hot chicks it once was, is still where some of the biggest news and newest demos are shown.
If New Vegas comes out in late August like I heard, expect a good lenght demo, big news, complete walkthrough of the first few missions, date announcement and so on.
If it's in November like I expected since I heard "somewhere in 2010" expect two to five new missions being shown, a brand new trailer and a promise that the game still need some polishing.
In any case, please, do NOT use G4 for your coverage. Seriously, open IGN and refresh every hour or so and you'll be overwhelmed with information.
Now I wonder if it will be Natal compatible on 360?