The Los Angeles Times talked to Iron Man 2 director Jon Favreau, who says they have started working on the sequel:
I just talked to Jon Favreau a bit ago, and he said that while there hasn’t been an formal announcement on the sequel to “Iron Man,” he’s begun developing it. “We’re working on it now,” he said, “which hasn’t been officially announced. It will be released in 2010.”
Marvel Studios and Paramount Pictures are targeting an April 30, 2010 release date in order to be the first film to kick-off that summer, much like Iron Man did on May 2nd of this year.
Justin Theroux (Tropic Thunder) is writing the script and in addition to Robert Downey Jr., the first film’s co-stars Gwyneth Paltrow and Terrence Howard are expected to return as well.
Source: Los Angeles Times