It's been confirmed by Ed Bouchette that Willie Colon will be moving from right tackle to left guard, in an article which you can read in its entirety here, which means both Steelers rookies David DeCastro and Mike Adams will have a chance to start early.
"The move from his spot at right tackle, paves the way for their top two draft choices to start immediately. David DeCastro will be put at right guard and Mike Adams at left tackle. Marcus Gilbert, who started at right tackle last year as a rookie after Colon was injured in the opener, was scheduled to move to left tackle but likely will stay at right tackle now."