Well, it seems like I was right on the money. In my preview two weeks ago for Leviathan, Mass Effect 3's first true story-based DLC, I guessed that it would be releasing within the "next two months". Okay, okay, so I left myself with a lot of leeway.
BioWare has revealed that Leviathan will be available on PS3, Xbox 360, and PC on August 28 in North America and on PS3 in Europe on August 29th.
In Leviathan, Shepard must investigate the titular rumor of the Leviathan, which has been said to have defeated a Reaper. To do this, Shepard and his team must explore the oceans of another world, at times rummaging along the bottom of the ocean floor with an underwater Atlas mech. The developers promise that this DLC will delve into the origins of the Reapers.