korell: If you got the game on PC then you didn't need to get it at release to get the Sewers DLC. The Sewers DLC is given to all newly purchased copies, so being as the game is Steamworks, you can only buy it new on PC. (I did read that there was some issue on early release where it disappeared from some users, but that seems resolved). For console versions, there was a Sewers DLC single use code for them to use, so those who got second hand/rental copies of the game couldn't use the Sewers DLC.
All they provided was some more mutant killing areas anyway, and one of the sewers (Plateau Sewers if I remember right) had a large arena type battle area near the start.
F1ach: I got the 360 version, sewers was free for pre orders here, iirc I may have got 2 codes on preorder, one was definately a sewer system, maybe the other was for some sort of LTD Edition of the game, but I seem to remember 2 codes for some reason, could be mixing the second one with some other game though..
A colleague at work got the 360 version (not pre-order, was in a bargain bin actually) and he had a sewers code in the box as it was not second hand.
There was some pre-order DLC though, the Anarchy Edition bonus items (Fists of Rage, Double Barrel Shotgun, Rat Rod vehicle, Crimson Elite Armour). However, I got that for PC in my recent retail disc purchase, I think because there are still copies of the game that contain the pre-order bonuses that didn't sell at the time.