The wonderfully twisted passages of the internet have unearthed an Asian product page advertising an April release for a Gears of War 2: Dark Corners add-on.
The image, posted on Joystiq, was discovered by a Gears of War forum user in a now defunct thread, fueled by former Kotaku guest editor Geoff Keighley's Twitter entry regarding Gears of War 2 level announcements. If the rumor holds true, this would see two Gears of War 2 add-ons released within a month of each other, with Snowblind due out on March 31st. It doesn't seem like a wise idea, releasing more maps while people are still getting used to the last batch, so this could very well be an extension of the single player campaign as well.
Hopefully we'll find out more as the Game Developers Conference continues. Until then, strictly in the realm of rumors.