Did Reggie Bush push his quarterback? Yes. It's visible on replays, and in the papers Bush readily admitted to it.
But so what? For starters, the push isn't what helped Leinart score. I've watched the replay a million times now and what got Leinart in was when he had that awkward roll on his back and slid towards the sideline, the whole time keeping his feet moving (I guess those ballroom dancing classes helped?). Bush did no more than push Leinart into the pile where he was initially stopped. Once Bush fell out of contact is when Leinart rolled and scored.
Besides that, the rule is never called. Im my lifetime I have yet to see that call made. And in the name of equity, Notre Dame was guilty of no less than three similar penalties on Brady Quinn's crucial third down quarterback sneaks throughout the game. A general rule of officiating is to uniformly apply one's calls to all game situations. Think about baseball, and how umpires work. The players and fans don't mind so much whatever strike zone an umpire decides to call, so much as he remains consistent so everyone knows the boundaries.
If during that final play officials had called the push (for the first time in what, 30 years?), it would have been a break from how they had called the game. Notre Dame was no less guilty of similar offenses at several spots in the same game, without any hint of a flag going against them. So why call it there, on the game's final play?
Lastly, I'm a spirit of the law type guy. I hunch the reason the rule was first put in the books was to discourage teams from attempting rugby-style scrums, which are not really how American football is played and can also cause a lot of injury. What Bush did, at best, was give a quick assist to a guy colliding with a goal line pileup. By the letter of the rule, he was wrong, but not in its spirit.
Notre Dame coach Charlie Weis actually called it a heads-up play, and hoped his players would do the same.