The North American beta includes the "Frozen Dam" multiplayer stage, and includes all three multiplayer modes from the game. Guerrilla Warfare is a standard team deathmatch type, Warzone gives randomly assigned mini-missions, and Operations provides linear mini-missions that tell a story. The beta will also include the offline mode Botzone.
Though a closed beta has been in operation since October, it was primarily to test gameplay and lacked some features. This version will be adding 3D support, and as an open beta it's likely meant to stress test the servers. Just be prepared to get beaten down by the players with months of closed beta experience under their belts.
That's not that many guys. http://www.shacknews.com/screenshots.x?gallery=16644&game_id=8924&id=172277#img172277
I had more than that on Contra.
But ... these guys are in 3D, so awesome! ^^