Battlefield 3 and, thus, the "Back to Karkand" map pack DLC, though each retailer will also have some different additional bonuses.
GameStop is offering the (timed exclusive) "Physical Warfare" DLC and PowerUp Rewards members will also snag a copy of the Prima strategy guide. Best Buy pre-orders will receive the "SPECACT Kit," a package of eight multiplayer skins. Finally, Amazon pre-orders include a pack of exclusive multiplayer dog tags.
Sadly not included with any of the pre-orders: Free time.