The Gears demo was co-op and pretty fantastic.
I will post more detailed impressions, along with impressions of Spider-Man: Web of Shadows, Madworld, Wii Music, H.A.W.X., Saints Row II and moar when I get home (I'm typing all of this on a Blackberry).
I'm attending the Harmonix Rock Band 2 presentation this morning, but there's no way in hell they will match the Activision Guitar Hero presentation in spectacle. Harmonix has to win the hearts and minds of DMs and SMs this morning, because after Activision last night, Guitar Hero won a lot of wallets.