The Commercial Vehicle Safety Alliance has released the statistics from Roadcheck 2009. According to their press release at: http://www.cvsa.org/news/2009_press.aspx , compliance levels set new records, with over 95% of all drivers, and over 80% of all vehicles passing inspection.
The press release also details reasons why drivers and vehicles were placed out of service.
For vehicles put out of service, over 50% of the total placed out of service were for brake adjustment, or braking system faults. For drivers placed out of service, over 50% were for HOS violations.
The press release makes interesting reading, particularly the tables at the end that break down violations by type.