Currently we are unable to confirm the exact time of the opening of the Blue Mars open beta, but we can confirm that the game has been shut down and is being prepared for the "public beta" according to posts on the forums.
Blue Mars is the middle ground between Second Life and Planet Calypso. Where Planet Calypso acts as a game with a real money economy and Second Life allows all users to create content and change the world to their preferences, Blue Mars signs up and approves select developers to create content for their world. Standard users will have the ability to open up small shops and/or purchase living quarters (at the developer's preference) as well as interact with the games and events the ruling developer sets up for their players.
So that's about it. It's not as open and free as Second Life, it's not a virtual world acting like a game a la Planet Calypso. It's something of a middle ground between the two, and it's also running on CryEngine 2 to make sure it stays up to date with their graphics. How will it do out in the open world? Guess we'll find out soon enough.