It's the kind of deal given to a player expected to play across the line, but with a tenuous situation involving Willie Colon and his contract, the move could indicate Colon's time is about to end. The Steelers drafted David DeCastro in 2012 with the hopes of having him start at right guard from the start of Organized Team Activities. DeCastro was well on his way until a knee injury thrust Foster back into the starting lineup from where he was in 2011.
The Steelers had decided to move Colon to left guard from right tackle, but the results were up and down. Colon is owed $7.5 million for the 2013 season, and may simply be too expensive to keep. The signing of Foster, along with the likely need to bring in a back-up center (assuming they won't ask Foster to fill that role as well) shows the direction in which the Steelers offensive line is heading.
Foster was an undrafted free agent in 2009 out of Tennessee. There were reports he was in contract discussions with Arizona until he signed with Pittsburgh.