(Also see the Wikipedia entry on Arar.)
This is further evidence of the TSA's failure to competently maintain the no-fly list.
UPDATE (October 20, 2006): Ed Brayton has discussed this story today.
UPDATE (January 23, 2007): The U.S. Attorney General and head of Homeland Security are both insisting that Arar remain on the no-fly list for reasons which they have disclosed only to officials in Canada. The Canadians don't think those reasons make any sense. My guess is that they think somebody they sent off to be tortured might have a beef with the people who did it to him.