Gears of War 2 Title Update 4, and Epic's Rod "Redbeard" Fergusson has detailed over on the game's official boards just what to expect when they take the knife to the game.
For starters, you'll accrue multiplayer XP from playing Horde mode, and also be able to restart a public Horde match from the last wave you passed, rather than wave 1.
Also added (in preparation for the upcoming Dark Corners expansion) will be 13 new achievements, worth a total of 325 points. But, uh, you'll need Dark Corners to get them.
You can get the finer details over on Epic's boards.