Well the actual DLC was not leaked, but info about it was unintentionally released during a PS3 patch. http://www.joystiq.com/2011/02/21/mass-effect-2-arrival-dlc-detailed-by-latest-patch/ It is obviously one of the DLC's Bioware stated to link between Mass Effect 2 and Mass Effect 3.
Yeah I heard about this and I can't wait for it. The conclusion to a fantastic game. But the start of a hopefully stellar third in the trilogy. By my imagining from the ME3 trailer and the title of the DLC, this will probably deal with the first attack of the ancient enemy (can't be bothered to make the spoiler tag) and how Commander Shepherd will organise the galaxy's defence against them. I'm just glad to be getting back into the shoes of Shep.
I hope it doesn't connect the games at all. I can't play DLC, and there are plenty of people that can't play DLC that wouldn't have as great of an experience. They did the same thing with the interactive comic. I got stuck with all the stupid default choices because I couldn't access it.