I see Ed Morrissey at Captain's Quarters has posted an excellent fiskinof Arkin's Able Danger post, "Arkin Denies What Pentagon Already Admits" at:
http://www.captainsquartersblog.com/mt/archives/005521.php
"Besides, Arkin appears to write this about four weeks too late. On September 1, the same Washington Post at which Arkin writes this reported that the Pentagon itself found three additional witnesses to the identification of not only Mohammed Atta as a potential AQ operative but the other lead 9/11 hijackers as well. It came as a stunning reversal after a week of denials from the Department of Defense..."