I know for a fact that that Black and White will be shown at E3 tomorrow, as a multiple of sites have confirmed it. But the question is in what way? Demo? Teaser Trailer? Full length trailer? etc.
Most likely it will be a full trailer, showing a couple of new things in it such as more towns, maybe a new pokemon?
Demo is less likely, but as the game comes out in September, there isn't a lot more time to show it off. So a demo could occur.
If it did I could see a route or two being shown, and in them some new pokemon if any are revealed at E3.
If I could have it my way I would want this to occur-
1.a good intro, going over the past decade and a half of pokemon
2. 2-3 minute trailer of the games
3. After the trailer the developers explain some of the new features, and offically reveal the starters.
4. Demo time. as I said before, 2 or so routes within the game that might show some new pokemon, not yet revealed even to Japanese audiences.
I know, wishful thinking. But who wouldn't want this sort of set up for E3?