William Connolley and I (both people who have worked professionally on sea ice) are arranging our bet regarding this year's minimum sea ice cover. I've already mentioned a bit about my prediction, which is for this year's average sea ice extent for September, in the Arctic, as computed by the NSIDC, to be about 4.92 million km^2. To the extent that my working model is good, the standard error of that estimate is about 0.5 million km^2.
In making that prediction, I'm taking the approach that the last two years' dramatic low covers were part of a continuing process of decline in the ice pack, though a fair portion of it still being peculiar circumstances of Arctic weather in summer/fall 2007 and 2008. This is not the usual view in the field, where the most common take is that 2007 represents a step change in the system -- one sort of thing going on before 2007, massive change in 2007, and things should continue more or less similarly for a number of years. (Until the next step change.)
William is taking the view that 2007 and 2008 were just bizarre years, not a fundamental change as most are taking it. The reasonable prediction in his view is to take the years 1979-2006, run a straight line through them, and use that as your basis. I've finally done the math for it (not at all difficult in the spreadsheet but I've been running around), and his prediction for 2009 is 5.84 million km^2 (also with about 0.5 million km^2 standard error -- but I'm rounding down for his figure, and rounding up for mine. That is, my scheme makes a better fit than his.).
So William, how about even quatloos over/under 5.38 million km^2? If it's under, you pay me 50, if over, I pay you 50? If your approach is right, 84% of the time you'll win this. If mine is, it's 84% of the time that I'll win. So, as you advised, it's a wager we each think is biased in our favor.