Apparently the Department of Defense believes that PS3s are a better value when it comes to supercomputers than IBM products specifically designed for the purpose. Granted recent price drops probably didn't hurt in justifying a 2,200 console order either.
This isn't the first time that the DoD is using PS3 consoles for supercomputing. In fact, these 2,200 units are going to be added to an existing Linux cluster of 336 PS3s used by the United States Air Force. According to Justification Review Documents, the purchase is all about getting the best value out the DoD's budget: