"For each day that the high temperature in your hometown is at least 1 degree Fahrenheit above average, as listed by Weather Underground, you owe me $25. For each day that it is at least 1 degree Fahrenheit below average, I owe you $25."
He's trying to address recent statements by some conservatives, paraphrased as "It's cold this summer here in Minneapolis, so global warming must be wrong." That's a bit of a strawman, but from the Power Line blog post, there really is this sense of local vs global perspective.
Well, it's actually a pretty silly challenge to a pretty silly statement. No serious GW skeptic I've heard contests that their is warming on a global scale, but argues against the magnitude or, more commonly, the cause of the warming (human vs not). The statistical challenge here only addresses whether there is warming, and even there is rigged to win even if there were no real global warming, because of the urban heat island effect. Most of the thermometers started out in rural areas, or in fields outside of towns, and cities were built around them. Areas around pavement are warmer than the surrounding areas, so there would be a measured warming trend due to development, not due to atmospherics.
A better bet would involve predicting the global temperature for, say, 5 years from now (along with the uncertainty). Each side puts in their prediction, and pays $1 times the ratio of the posterior probabilities for the two models, P(M1)/P(M2). Would anyone take a bet like that?