Battlefield 3's pre-order incentive is a free map pack called BF3: Back to Karkand. In order to receive this pack for free you must pre-order the Limited Edition version of the game. The multiplayer pack features popular maps from Battlefield 2 recreated using the Frostbite 2 engine. The pack also includes BF2 weapons, vehicles and new achievements/trophies.
Battlefield 3 is scheduled to launch sometime this fall for the PC, Xbox 360, and PS3.