The rule book states that even if the receiver's feet are down in the endzone, if the ball does not cross the plane, it is not a TD. After extensive review, you could probably just about argue it did cross the plane in the Ravens-Steelers game, but it was too close to call so the original decision on the field should have stood. Watch this vid http://www.nfl.com/videos?videoId=09000d5d80d601d1
at about 0:15 you will see the ball appearing to cross the plane. Hope that clears that one up, it's strange how video replay decisions can lead to even more controversy than not using them sometimes!