Personally, knowing the Spurs and having watched them forever, I wouldn't consider this an "upset" (I picked the Spurs in 6 or 7) because these teams were only 5 wins apart going into the playoffs...
But let's not kid ourselves, everyone and their mom (in the media and ISH) was picking the Mavs in this series.
Nobody took the Spurs seriously, but they have the talent, experience, and solid coaching to be a serious threat.