The court will hold a hearing "en banc" (meaning in private with a full panel of judges [thanks matthew]) on December 9. Unusually, the decision was made "sua sponte" or on the judges' own initiative, rather than in response to any motions that had been filed. Eva Luna tells me that she can't think of another case of this magnitude where the court has decided to go back and re-examine its previous decision on its own.

The CCR, which filed the case on behalf of Mr. Arar, summarizes his treatment:

Based on faulty information, Mr. Arar was detained as he was changing planes at JFK airport on his way home to Canada from a family vacation. A Canadian citizen, he pled with officials not to send him to Syria, the country of his birth, because he would be tortured there.

After nearly two weeks in New York, with access to counsel and the court obstructed, he was flown to Jordan on a chartered jet in the middle of the night and taken by land to Syria. Mr. Arar was tortured, interrogated and kept in a 3x6x7-foot underground cell for a year until the Syrian government, finding no connections to terrorism, released him home to Canada.

However, in response to the CCR's filing, the 2nd Circuit ruled in June 2008:

that Mr. Arar’s constitutional claims for being sent to Syria to be tortured and arbitrarily detained could not be redressed because Congress already provided a remedy by permitting foreign citizens to petition a court to review their removal orders, even though in this case the U.S. officials prevented Mr. Arar from doing so. The court also found these claims could not be heard because they would interfere with U.S. foreign relations and impede national security.

The CCR also links the original order (pdf).