The new DLC for Gears of War 3 entitled "Horde Command Pack" was supposed to have been released today, but due to some unspecified technical issues the release has been delayed.
Epic worded it as follow on the official forums:
"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."