The Gears of War 3 Horde Command Pack, the first piece of DLC set for release for the co-op game, has been delayed at the last minute due to technical glitch according to the game's community manager raczilla. The content, which is priced at 800 
, was set to go live today.
The scheduled Horde Command Pack release for Gears of War 3 is temporarily delayed due to a technical issue. We apologize to those of you who are anxiously awaiting to play. We know this is disappointing news as we’ve seen a lot of posts today showing excitement for this content. We have folks working around the clock to resolve this issue ASAP so you can all play and enjoy the Command Pack.
The Epic Games Community team will stick around and keep you posted with any further information. Stay tuned for updates!
The Horde Command Pack is set to feature 3 new maps, 4 new fortification upgrades for Horde mode, 3 new characters and 2 new weapon skins.
It seems that DLC delays are all the rage these days, perhaps as DLC becomes more and more complicated and not just "maps." Recently Space Marine on the Xbox 360 suffered a similar delay for it's Exterminatus DLC.
Update: Epic's VP Mark Rein tweeted that the content should now be live! Glad they got that technical issue solved quickly.
Source: Forums.epicgames.com