The "undecideds break for the challenger" is mostly true in local elections. In presidential elections, undecideds usually break for the incumbent.
Al Gore surged late in 2000 not because of undecideds breaking for him, but because of the DUI thing. It's estimated that as many as 4 million evangelicals stayed home on election day because of that.
This time around, the evangelicals are pumped for Bush. It seems unlikely the Dems could pull off an "October Surprise" because Bush is a known quantity and they've been throwing the kitchen sink at him for 4 years.
I'm hanging my hat on your last comment in particular. It's what I was telling my friends in June when they were taunting me with polls saying Kerry was ahead. Bush had just spent the last year being based without response by the Dem candidates and their media allies, yet Kerry never built a significant lead. I'm hoping the same logic still applies. I'd sure like to see W consistently hit 50% in some of these polls soon, though.