Gametrailers is kind of getting hit hard for bandwidth right now, so be patient. These links are to the HD videos. If you want the SD, there is a button on the bottom left of the video player that says "SD".
Warhammer 40k: Space Marine - PS3 and XBox360. You take control of a 4 man squad (one at a time, when you switch is unclear) consisting of a Tactical Marine, Assault Marine, Devastator Marine, and a Sergeant. (Wikipedia)
Warhammer 40k: Dark Millenium Online - PC. In the video we see players in control of Orks, Chaos Marines, Guardsmen, and Space Marines. Possibly heretics (traitor guard). Multiple different chapters. Players are also shown in control of Deff Coptas and Bikes.
This first taste of the 40k MMO is killing me...it's supposed to be out in 2012. Hopefully we'll more information over the next 2 days on both games. (E3 2010 started today and goes until Thursday)