Before the debates, the political science literature was clear: presidential debates do not matter. Even after the first debate when people started to worry whether Obama had blown the election in a single night, I reiterated the idea that the debates still do not matter - even though it was not a popular idea at the time.
Now I have some additional insight on why:
1) There is more than one of them. If the front runner blows the first debate, then there is still plenty of time to figure out what went wrong, take corrective action, and undo the damage with strong performances the next time around. This is what happened this year with solid showing from Obama in debates number 2 & 3, as well as Biden's fantastic performance in the VP debate.
This is also what happened in the Kennedy-Nixon debates. In the first debate, Kennedy looked presidential and Nixon looked perturbed. Nixon's performances got stronger as time went on and the overall change in the polls was negligible.
The exception to this rule is the Reagan-Carter debate in 1980, mainly because there was only one debate and it was a week before the election. But it probably was not decisive because...
2) Even if one candidate experiences a bounce, it may not be enough to change the ultimate outcome of the election. Reagan was probably going to win anyways in 1980. He just won by larger margin than he would have otherwise. Obama will still (most likely) hold on for another term. He will probably win by far fewer electoral votes (let's day 290 instead of 347) than he would have if he landed the knockout blow at the first debate.
3) A week is a long time in politics. A lot can happen. Breaking news and campaign tactics can make a difference. The heralded "October Surprise" can make an appearance. Given enough time, the debates are ultimately forgotten about. Just like the conventions before them and then primaries before that. Shake the Etch-a-sketch.
4) Finally - No matter what each campaign says or does, polls tend to revert to the mean. We are today where we were most of the summer. Same place that we were before the conventions. A close election with a narrow lead for the incumbent.