According to Geoff Boucher of the LA Times Christopher Nolan will start shooting Batman 3 in March of 2011. Boucher was talking about asking Nolan about Batman 3 in a upcoming interview, and here is what he said,
I plan to ask him about the third Batman film, which starts shooting in March, and his plans to produce a Superman revival with his partner and wife, Emma Thomas.
This is the first time we've heard of any kind of start date on the film. Either that was the first month to pop in head, or he has information that the studio wants to get the film into production by March of next year. The film already has a release date of July 20th 2012, and they gotta start shooting it sometime... why not March?