The Heisman thing would be completely up to the Heisman Trophy Trust who gives it out (formerly done by the Downtown Athletic Club).
They have nothing to do with the NCAA, so it's not like the NCAA or USC can tell them they have to take it back and give it to someone else. Would they? Who knows...I mean, if you're Vince Young, would you want to have it given to you now? By default? I would think not.
I could see them refusing to give it to someone for things discovered prior to the actual ceremony, but if they haven't taken OJ Simpson's back from him, I don't see them taking back Bush's now.