Eye witnesses put Agent Coulson, even at the time a relatively highly placed field agent of the recently renamed SHIELD, at the scene and having briefed Mr. Stark shortly before the press conference- which does lend some credence to the idea of a cover up.

We also need to put the moment in context- Stark Industries had recently, and very publicly, announced their intention to exit the arms industry. By selling himself as Iron Man, Mr. Stark could both help his company's flagging fortunes, as well as potentially be the beneficiary of lucarative contracts.

All that said, I personally think he was in the suit; there's too much evidence indincating that to be the case! However, was he Iron Man? I believe that the suit was effectively an autonomous drone. Basically, Stark was trying to do an end-run around the Dreyfus Act; compare with OmniCorp's 'RoboCop' project.