As previously speculated Battlefield 3 will not appear on Valve's Steam service. The reason for this is the same reason that has seen Dragon Age II and Crysis 2 removed from Steam. EA now confirms the decision to not publish Battlefield 3 on Steam in their forums.
"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."
As far as we understand there is nothing in Steam's terms of service that restrict the release of patches, however Steam does not allow games where sales of DLC is only made available in-game (thus cutting out revenue from Valve and Steam).
It would seem as though EA and Valve are involved in some kind of tug-of-war over DLC revenues. Unfortunately gamers are being caught in the crossfire.