I believe that the American government deliberately is not doing anything about it so they keep the population on the alert and in fear and this way easily justify the tremendous amount of money they spend on military operations and equipment.
I am not a great fan of conspiracy theories, but this case looks very suspicious. I don't believe that the most powerful army in the world backed by the most powerful intelligence agency are not capable of finding a man hiding in a cave.
They even seem to encourage and help the circulation of the regular "videocasts" Osama is sending to the world.