I've heard that Forrest jaw was broken. After watching a replay of the fight, it almost looks like he gets it broken when he goes down the first time. He sort of puts his hand up and it just looks odd. Then he gets blasted two or three time "on the button", before Silva extends his hand to help him up. The finishing punch did not look anymore vicious than the others, but Forrest looked hurt, not dazed or out of it, but hurt.
If that was the case and Forrest got his jaw broken early, he showed a lot of heart in trying to fight on. For what its worth, Silva out classed him and knew it.