Military.com had a chat with Terrence Howard's who plays Col. James 'Rhodey' Rhodes in Iron Man, the actor confirmed that he will begin filming for Iron Man 2 in March of 2009. The sequel has already been scheduled for a release on April 30, 2010. Jon Favreau is expected to direct again, with Robert Downey Jr. and Gwyneth Paltrow returning as well.
UPDATE:
Jon Favreau wrote on his MySpace page saying the scheduled release date for Iron Man 2 as "unrealistic", and there is no start date for filming as there no writers attached at this time.
Neither Robert [Downey Jr.] nor I were consulted about this and we are both concerned about how realistic the date is in light of the fact that we have no script, story or writers hired yet.