You can register your Pilot Name right now. Get your very rare name before anyone else does. I got a really good one. :)
The FAQ is located here. Basically, it's a PC exclusive Free to Play MMO. MWO is running an "offset timeline" to BattleTech and in 3049. No melee combat, no vehicles yet, Mech team-based. The Atlas, Hunchbask and Jenner Mech designs have been confirmed, and the Clans will be making an appearance.
* UPDATE:PC Gamer has an "exclusive" article on MWO.
Oh right! DC Universe Online is going F2P tomorrow too. I'll probably play it for 1 or 2 weeks before getting bored, but it should be interesting.