Gears of War 3 was set to see its first downloadable content release launch today, the Horde Command pack. "Was" being the operative word in that sentence. An update in the Epic Games forums confirms a "temporary delay," citing "a technical issue" as the cause.
When it arrives, the add-on will bring three new Horde and Beast mode maps, three playable characters, two weapon skins and upgrades to the Horde mode fortification mechanics into the game. Epic promises that "we have folks working around the clock to resolve this issue ASAP," but there's nothing more specific about the timeframe for the eventual release.
Source: Epic Games