My theory is as follows: The political impact of this story is only now starting to appear.
The Al-Qa Qaa story broke last Monday in the New York Times and was big news throughout the week. The first tracking polls that would have included polling from Mon-Wed came out yesterday. As is usual with big news events it takes a few days for the negative news on Bush to affect people's voting preference. And this year there are fewer undecideds than there often is the week before an election - limiting the amount of movement we might see from a big news event.
However, today (Fri) is the beginning of the period where we could see some of that re-appraisal (for those who support Bush) and some drift to Kerry.
TIPP and Zogby confirm a drift towards Kerry. If this continues we will know that it has had a political impact. The fact that it hasn't significantly altered the polls yet should not be taken to mean it won't have an effect. It could likely still have a significant effect on the election (3-4 points in Kerry's favor).