I just googled for pics of the game and found this. As explained, the touchdown was made with him breaking the plane in the air. Click on the picture, "Photo Gallery," on right of page. Then click on picture #15.
http://www.latimes.com/sports/football/nfl/la-sp-penner6feb06,1,5527173.story?coll=la-headlines-sports-nfl
Yeah. He broke the plane. The ball didn't.
It's no big deal, though. The Steelers would have won anyway.