Electronic Arts revealed the Back to Karkand DLC bonus for Battlefield 3 back in February, a multiplayer map pack included with all pre-ordered copies of the game. There's more free stuff to be had though, as a new Battlefield Blog post reveals the retailer-specific bonuses you can nab alongside Karkand later this year.
GameStop pre-orders come with the Physical Warfare pack, which was previously announced as a pre-order offer from UK retailers GAME and Gamestation UK. The pack adds four weapons/accessories to BF3: Type 88 LMG, DAO-12 shotgun, flechette ammo for the DAO-12 and a flash suppressor for the SKS sniper rifle. The shotgun previously appeared in Battlefield 2 and the Type 88 LMG, in Bad Company 2.
From the sound of things, that's the best of the bonuses too. Best Buy pre-orders come with the SPECACT Kit, which adds eight additional multiplayer skins to the game. Finally, an Amazon pre-order nets you an exclusive set of dogtags for multiplayer (the game features custom dogtags, FYI).
Free weapons vs. free skins vs. free custom dogtags. There's an easy choice to make.
Source: Battlefield 3 Blog