As a reminder, here's the Steelers' 2011 Preseason Schedule. They'll play the Washington Redskins during Week 1 on August 12 at 7:30, then the Philadelphia Eagles on August 18 (at 8:00 PM on FOX). They'll then take on the Atlanta Falcons at some point between August 27 at 7:30. They'll wrap things up with a game against the Carolina Panthers on either September 1 at 8:00.
This, of course, assumes that the NFL Lockout will finish in time for all the games to be played. It would be surprising if it didn't, simply because the preseason is really lucrative. The NFL will have plenty of incentive to reach an agreement with the players if things go on much longer.
If the lockout lasts longer than, say, another week, it might be difficult for the NFL to play a full preseason slate. There will have to be a free agency period, and then, of course, training camp, where teams will have to familiarize all their new players with their system. It could get frenzied, and if the lockout continues, some preseason games may have to be cut.