Multiplayer characters are headed to Earth, according to Xbox team member Larry Hryb, on his blog for the XBL store, Major Nelson. This comes as a confirmation of the DLC rumored along with the Extended Cut's completion last month, as leaked by a Reddit user.
The posting doesn't share any details, but let's recap. According the the Reddit user who leaked the information, there would be three new maps: Vancouver, Rio, and London. Vancouver and London were both in the main game, so they may be different takes on those maps. Rio wasn't in the single-player game, so, assuming this information is accurate, it's probably just a sweet vacation spot for squads.
Also purported to be in the DLC pack was three new weapons: the Piranha assault shotgun, Acolyte pistol, and Typhoon light machine gun. No details here, but weapons of the previous pack seemed to follow a sort of theme, so we may see that here.
The largest bonus fabled to be in the Earth DLC is the addition of six human classes. The function they are supposed to have isn't clear, but the classes are named Destroyer, Paladin, Demolisher, Slayer, Shadow, and Fury.
Since there are six player classes in the game, it's safe to assume that, if this DLC pack is authentic, those six would add one new human to every class in the game. The six classes are Adept, Soldier, Engineer, Slayer, Shadow, and Vanguard, so if you line up the classes, it seems like their order might match up. At very least, Shadow and Infiltrator seem correlated.
BioWare hasn't confirmed this information, but Major Nelson places the release of the Earth DLC on Xbox 360 for July 17th, exactly one week from today. Mass Effect 3 is on the Xbox 360, PlayStation 3, and PC.