Funny, but essentially the exact same thing just happend with the same exact umpire, Gerry Armstrong, today during the 1st set of the Safin-Berdych match. Safin hit a shot that landed near the baseline, it was called out by the linesperson, the umpire overruled saying that he saw the ball skid off of the line so therefore the shot was good, Berdych appealed saying he still thinks the ball was out, the umpire goes to inspect the mark but first says there's no mark so he was right in that the ball skidded off of the line so it was good, Berdych then calls the linesperson over to find a mark which the linesperson does, then the umpire changes his call to the ball being out. This is basically the same thing that happened during the Robredo-Kohlschreiber match yesterday with the same umpire, yet he accepted a player's appeal again and changed his call once again. However, the big difference is that in this case, Safin didn't make a stink about it like Robredo did and accepted the final call of "out" and losing the point. He just went on with his business and served the next point without as much as a peep out of him. The difference in reactions between Robredo and Safin were huge when both were presented with the same exact predicament.
I think this is why they still should use Shot Spot even on clay because in both cases Shot Spot showed the ball to be clearly out. Sometimes it's hard to find the right mark or a clear mark on clay.