The Government Accountability Office has released Thousands of Civilian Agency Contractors Abuse the Federal Tax System with Little Consequence (GAO-05-637) (91 pages) (6/16).  From the 1-page highlights:

FMS and IRS records showed that about 33,000 civilian agency contractors owed over $3 billion in unpaid federal taxes as of September 30, 2004.  All 50 civilian agency contractors we investigated had abusive and potentially criminal activity.