The odds I saw for the second match were about 2:3 favoring AlphaGo; my guess is the odds moving forward will be more like 1:4 favoring AlphaGo (but probably it should be closer to something like 1:9).
if we're looking at an AI player that is better than a human can be.
This is my estimation.
Lee Sedol has now won the fourth game, which makes this very improbable. I still think AlphaGo is better than him, but this basically means that its competence can still be measured on a human scale.
There have been a couple of brief discussions of this in the Open Thread, but it seems likely to generate more so here's a place for it.
The original paper in Nature about AlphaGo.
Google Asia Pacific blog, where results will be posted. DeepMind's YouTube channel, where the games are being live-streamed.
Discussion on Hacker News after AlphaGo's win of the first game.