And looking at how he used up his time much sooner, he was more cautious today. He still lost and probably also took a psychological hit, so now my estimate of chances of Lee Sedol winning the whole match went down to ~5%.
Ignoring psychology and just looking at the results:
Delta-function prior at p=1/2 -- i.e., completely ignore the first two games and assume they're equally matched. Lee Sedol wins 12.5% of the time.
Laplace's law of succession gives a point estimate of 1/4 for Lee Sedol's win probability now. That means Lee Sedol wins about 1.6% of the time. [EDITED to add:] Er, no, actually if you're using the rule of succession you should apply it afresh after each game, and then the result is the same as with a uniform prior on [0,1] as in #3 below. Thanks to Unnamed
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.