The EA Forums have a confirmation of widespread suspicions that Battlefield 3 will not be sold through Valve's Steam platform (thanks Planet Battlefield). As was theorized, there is a terms of service conflict at the root of this because DICE's military shooter will include the ability to purchase DLC from within the game:

Gamers can pre-order Battlefield 3 at Origin.com as well as over 100 digital retailers worldwide. EA offers games to all major download services. Unfortunately, Steam has adopted a set of restrictive terms of service which limit how developers interact with customers to deliver patches and other downloadable content. No other download service has adopted these practices.

We are intent on providing Battlefield 3 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. We hope to work out an agreement where Steam can carry Battlefield 3; meanwhile, gamers can pick from the more than 100 digital retailers listed at http://www.battlefield.com/digitaldownload, or go to http://www.origin.com.