[UPDATE]: I just found a respone from Valve on the forums, stating that the problem is that they are out of Mad Moxxi DLC codes right now. It could be as late as Monday before they get more. They still have codes for the other DLC that requires them, The Zombie Island of Dr. Ned. The game itself should be playable now, though.
If any of you are tempted by Borderlands: GOTY Edition being 75% off ($7.50!) this weekend, you may want hold off for the time being. Why? Because currently, almost everyone who has bought it is having some major issues right now, as they apparently cannot connect to the key server to obtain their CD keys. I've been looking around the steam forums and have been seeing the exact same thing happening to them, as well.
I'm posting this here because as I speak, there seems to be no response from either Valve, Gearbox, or 2K Games about this issue, and I am hoping that the more people know about this, the quicker this gets fixed. After all, we just want to play the game that we just bought. Please spread the word about this.