I'm going out on a limb and calling that Nintendo will "win" E3. With Zelda, Vitality Sensor, and Pokemon Black/White, they're already looking great, but I expect a few more big announcements from third parties and at least one big announcement from Nintendo.
Close behind will be Sony, they just have too much that they can show, we're going to see a variety of AAA games including but not limited to Resistance 3, Killzone 3, inFamous 2, LBP 2, Motorstorm 3, The Last Guardian, Gran Turismo 5, and anything 3rd party.
Microsoft will be last, unless they somehow buy timed exclusivity for Call of Duty: Black Ops. Their games aren't big enough, and Natal isn't going to be that successful unless it can produce hard light like the X-Men's Danger Room.
Currently playing: Uncharted: Drake's Fortune, NBA2k11, Metal Gear Solid, Picross 3d