Wired's Dangar Room blog and TG Daily report that DARPA officials are only allowing 11 teams into this Saturday’s Urban Challenge race down from the expected 20 qualifying teams. It states that Dr. Tony Tether, the head of DARPA, said the decision was based mainly on safety and whether the robots would impede other vehicles from reaching their destination.
Wikipedia has more details of the race and selection process and Wired's Dangar Room Blog has a large amount of coverage.