I've alluded to this in the past, but I think it's clear that McCain's attacks clearly got Obama off his game plan and on the defensive.
But interestingly, I don't think that directly happened.
It was really a three-step process...
1. McCain's campaign attacked Obama, hoping to draw him into a fight (and away from his "change," "post-partisan" image).
2. Democrats, fearing he would become Dukakis, Kerry, et al., criticized Obama for not responding. Obama could not afford to allow this to become his narrative.
3. Obama changed his strategy, attacked back, made decisions to avoid being attacked (like picking Biden to cover his inexperience on foreign policy flank). This ultimately turned him into a typical "politician".