I actually prefer the way he handled it today - if the server hits a 2nd serve, called out, challenged and called good, I think he should get a 2nd serve. It doesn't make sense to replay the whole 'point' since no real disadvantage was caused to the server.
davydenko successfully got a 2nd serve in, so why should he have had to get another one in?
A double fault is when you miss 2 serves in a row, he made a 2nd serve, so he shouldn't be given another 2nd serve because if he missed that one it would be a DF for missing only 1 serve in a row.
He made his serve, only reason that it wasn't allowed was a bad line judge call, why should he have to face the risk of having to make it again just because of a bad call? The fact that he had to is a disadvantage to the server isn't it?
a DF is 0/2 serves in, not 1/3