From a reader:
Slate assumed the probabilities for Ohio and Florida are independent, giving the president a 25% chance of winning both. This assumption is wrong – if polls are underepresenting Bush (or Kerry) votes in one state then they are likely to do the same in both. Note that Tradesports has a “Bush wins Ohio and Florida” contract. It has been trading between 35-42 over the weekend. Not 25 as predicted by Slate. The people at Tradesports are betting the outcome of the two races are not independant.