The excellent 'Starts with a Bang' science blog has a very good article about what an incredible stat his times are:
http://scienceblogs.com/startswithabang/
There's also this graph, which shows that he's still slowing up a bit at the end, though not nearly as much as in Beijing
http://i31.tinypic.com/miigw7.gif
Personally, I think he's clean, in part because he doesn't fit the model for dopers. He's been beating everyone in his age class consistently since the age of 16, and it doesn't make a lot of sense for anyone to take the risk of doping when they're head and shoulders above their competition.