All I saw was Jackson's (I've got to defend my fellow Gator, after all :)) arms go under the Steeler defender, I never saw the hands touch the defender's chest in a way that allowed Jackson to push off, or stopped the momemtum of the defender. And the defender doing the exact same thing to Jackson, in fact, you could have made a case for defensive pass interference on that play.
Jackson pushed the defender in the chest with his hand, and used that momentum to create separation. Other than a pick or blocking downfield before a catch, it's about as blatant as offensice pass interference gets. I don't see why people are complaining about the call (other than die-hard Seahawks fans, who need to comfort themselves by blaming anyone but their own team).