That is something we could and should do. It is, as you say, a humane and civilized thing to do. No matter how much kerfuffle got raised from such a statement- Dubya should get on national television and say that right into the camera. The lefties, the UN, the EU, the Arab world and PETA would probably go nuts but, it would be out there and nobody could say they didn't know what was coming to 'em when they got nuked.
The next best thing would have been (and I hope it happened) for Dubya to have told the Saudi leader he just had to his ranch in no uncertain terms that that is exactly what will happen and for him to go back and tell all his jihad, Islam brothers the news. That approach is more diplomatic and would avoid a lot of hell being raised, but less effective IMO. Bin Laden might just say "Bollocks! I don't believe it!" and detonate his nuke anyway. If Dubya did it on television, people would know he was serious.
That would have some powerful deterence value.