Below is Geek Nation's video package from the Iron Man 3 US junket. It features interviews with Robert Downey Jr. and Gwyneth Paltrow and the very enthusiastic Kevin McCarthy asks a few interesting questions. But without a doubt the highlight is Paltrow's response to being asked about a fourth movie around the 6 minute mark.
"I don’t think there’s going to be [an Iron Man 4]. I think we’re done with Iron Man."
If you watch the vid it certainly sounds like she's being sincere, and at least does believe that there won't be another one. Of course Downey Jr. has hinted as much too -- is it possible that Paltrow knows that he's declined to sign a new contract?
Iron Man 3 would definitely serve as a conclusion to the character's on-screen adventures..at least for the foreseeable future.