Not that huge an upset if you look at their records over more than, say, the last 2 months.
Roddick actually does not have a very good record against former or current top 10 players -he's improving of course but the really good guys who don't get tipped off balance by his serve and power, and/or have a big serve of their own, still give him major headaches. Good win for Tim though, Andy will have lots more chances and Tim probably won't.