Polamalu and Timmons are unrestricted free agents after the 2011 season and both players will be re-signed at some point, though it is not clear when. But, after restructuring six contracts and signing franchise player LaMarr Woodley to a six-year deal, the Steelers want to concentrate more on preparing their roster for the upcoming season with possible free-agent signings.
With the lack of depth at wide receiver suddenly a problem - the Steelers recently welcomed Jets wideout Jerricho Cotchery to camp as they dealt with injuries to Hines Ward, Limas Sweed and (perhaps most crucially) Emmanuel Sanders - the Steelers surely need to deal with potential problems with their 2011 roster.
However, I wouldn't count on the potential signings of role players like Cotchery keeping the Steelers from signing core players like Timmons and Polamalu before the season starts. (The Steelers don't discuss contract extensions with players during the season.) As Sam Wilmoth recently pointed out, Timmons, in particular, is a great candidate to try to sign to an extension because he's very good and his profile is still relatively low. Polamalu is perhaps a different case, in that he's older and injury-prone, but the Steelers don't have a replacement for him at the ready, and he's still a great player who's very visible both locally and nationally.