BF3Blog has received an anonymous report from inside Valve, that they are in late-stage talks with EA to bring Battlefield 3 to Steam. The rumor is that Battlefield 3 will be available just in time for the release of the game on October 25. According to the report, EA and Valve are very close to reaching an agreement regarding sales of DLC content, which is the main reason Battlefield 3 wasn't previously coming to Steam.
Valve requires that all DLC is sold through Steam, whereas EA prefers to sell it through their own channels. This was why Crysis 2 (another EA title) was pulled from Steam earlier this year. Many PC gamers have lobbied both EA and Valve to bring Battlefield 3 to Steam and it looks like this may just happen. Fingers crossed!