Iron Man director Jon Favreau cleared up on his MySpace page for the movie who worked on the Mark III armor:
The suit was designed by an artist named Phil Saunders who collaborated with Adi Granov based on the comic art of Granov. It evolved over many months and then was brought to Winston Studios to refine the design in order to build it as a working practical costume. Winston modeled and then fabricated the actual suit. It is difficult to identify a single person as the designer because they each contributed to the final design. The EW article correctly stated, however, that the suit was built by Winston. I hope this helps.
Iron Man is coming on May 2, 2008.
Source: the reel chris