This is big shit. When Weldon and his "Able Danger" song-and-dance first came over the wires, I didn't get too excited, because Weldon is one of those Chicken Littles who always get hissy and hyper over flaky stuff which often turns out to be so much hot air. But they got the 9/11 Commission people to admit that A)"Able Danger" was real, B) "Able Danger" *did* get some sort of hit on Atta and three of the other 9/11 shahids and that C) somebody on the 9/11 Commission staff had been briefed by "Able Danger".
This means that the 9/11 Commission people suppressed information contrary to their conclusions, and as Geraghty argues, that it is now much more highly likely that the Clinton-era "wall" between security agencies, the military, and other security elements was directly and squarely responsible for the success of the 9/11 attacks. It also might help explain why we haven't had a repeat of the attacks - that after "the wall" was dismantled, large-scale would-be terrorist conspiracies are visible enough in the States to be caught or disrupted before they get anywhere near their targets.
If I might go out on a limb here... it's possible that we now know who was responsible for the 9/11 failures, and her name is Jamie Gorelick.
Update: Devil's advocate viewpoint, via a Minuteman link.
Second update: Whoops. A weekend later, and it turns out that Weldon has mostly led us off a cliff. No mention of Atta's name to the Commission people, and it kind of sounds as if Weldon's source didn't put 2 and 2 together on the "making" of Atta until pretty late in the game. I should have stayed with my first instinct, which was to dismiss Weldon as an excitable flake. Oops.