Just in case you were wondering, Electronic Arts and Steam haven't made up yet. Their ongoing duel centers around two things: the existence of EA's rival Origin service, and Valve's policies regarding DLC on Steam. Mass Effect 3 is an EA game, and therefore will require Origin on PC, and will not be available for sale on Steam.
"During initial release Mass Effect 3 will be available on Origin and a number of other 3rd party digital retailers, but not on Steam at this time," BioWare confirmed. "Steam has adopted a set of restrictive terms of service which limit how developers interact with customers to deliver patches and other downloadable content."
"We are intent on providing Mass Effect to players with the best possible experience no matter where they purchase or play their game, and are happy to partner with any download service that does not restrict our ability to connect directly with our consumers," a forum post (via IGN) advises fans.
Origin will be required for all PC copies of Mass Effect 3, physical or digital. And, as previously reported, Mass Effect 3 will require a "one time, single authorization" for the single player game, but will not require a persistent connection unless playing multiplayer (for obvious reasons).
Mass Effect 3 will be available on March 6th.