Update: 1:33pm PST
Electronic Arts and DICE included a fourth map in the downloadable-only Battlefield 1943, but didn't make it available at the start. Instead, players have to collectively rack up at least 43 million kills to unlock the map. Fortunately for Battlefield 1943 players on Xbox 360, that time is very close -- in fact, the map will probably unlock tonight.
According to the kill count on the official Battlefield 1943 website, Xbox 360 players have accumulated 42,263,304 kills (comparatively, PlayStation 3 players have killed each other 22,066,012 times). At this pace, it's assured Coral Sea will unlock on Xbox Live in a matter of hours, while PlayStation Network-bound players will have to keep at it.
Update: 43 million kills achieved! The Coral Sea map should unlock later today. Congrats!
Which platform are you playing Battlefield 1943?