A hypothesis: this post is just sheer speculation, but...
We know that:
1. In two polls about the results of the third debate, John Kerry won by ~15%. A third poll showed Kerry leading by 1%, but the sample was very disproportionately Republican.
2. Since the third debate, John Kerry lost a couple of percentage points in the national polls - a small, barely statistically significant movement that is nevertheless there.
2. Since the third debate, John Kerry gained in the electoral college math, taking over President Bush and improving in the crucial swing states (Florida, Ohio, Pensylvania) - again, a very small and barely statistically significant movement.
I wonder if the simple explanation is that the Mary Cheney remark cost Kerry in the south but not elsewhere.
I think intuitively this explanation makes some sense. The south is the most socially conservative region of the country; more people think of homosexuality as something to be embarassed of, as something one does not talk about. More people would be offended by such a lack of manners. Lets not forget that a lot of people on TV arguing that Kerry's mention of Mary Cheney was bad are southern family-values types (James Dobson and so on).
And it would explain the situation: a shift to Bush in the south would cost Kerry in the polls but not in the electoral college math.
Or am I just reading into statistical noise here?