Check out Jack Pitney's analysis of the debate at National Review Online. In general, I think Jack's right. Kerry's substantive positions in the debate give the Bush people more material to paint him as a flip-flopper. On the other hand, I'm not sure how much good that does them. They already had more than enough to make the charge stick. On the other hand, Kerry's demeanor made him seem the more confident and resolute candidate.
If I were Kerry, I'd hammer Bush hard on the Iraqi troop numbers. Not only were they inflated, but they can be the wedge to show that Bush is out of touch on Iraq. This is his most effective defense against the flip-flop charge, showing the Bush is just too stubborn to face facts. Being consistently wrong doesn't minimize the fact that you are wrong.
I'd even twist the knife a bit by asking how many of the Iraqis we've trained are fighting with us and how many are fighting against us.