As most people likely expected EA have decided to make Origin a requirement to play Mass Effect 3 on PC. And in turn, Mass Effect 3, will not be available on Steam or make use of it in any way.
"Origin is required for the PC versions of Mass Effect 3, both physical and digital. Mass Effect 3 will require a one time, single authorization for the single player game. There is no limit to the number of installs. Playing Mass Effect 3 Multiplayer will require a constant connection."