If you got yourself a copy of the Limited Edition of Battlefield 3, then that free expansion pack that was promised inside your copy is coming real soon. Check out the new trailer coming from DICE showing one of the new maps aslo showcasing the added vehicles in the game. Thank god for that harrier type jet.
Back to Karkand will include 4 maps, 10 new weapons, 3 new vehicles, a new game mode called Conquest Assault, and a feature called assignments which is another way to unlock weapons. I believe most Battlefield 3 players have a copy of the limited Edition, but just in case you don't, you'll have to cough up $14.99 to get this expansion pack.
The expansion pack will come sometime in December with the Playstation 3 version getting it first a week earlier.
If you want a more detail about what certain weapons and vehicles are being included, check out the BF3 Blog.