No one, including himself, would argue that Bradley Birkenfeld, 44, is a saint. But at the same time, almost no one in the U.S. government would deny that Birkenfeld was absolutely essential to its landmark tax-evasion case against Swiss banking giant UBS. The former UBS employee turned whistle-blower exposed the previously hidden world of offshore tax shelters, which cheats the Treasury out of about $100 billion a year. Thanks to his insider information, UBS was fined $780 million, and it promised to exit entirely" from the U.S. tax-shelter business and to provide the names of thousands of American tax dodgers, from which hundreds of millions of dollars still might be collected. It also led to new tax treaties with the Swiss that should provide unprecedented tax information in civil cases and better access to such data in criminal cases. Considering Birkenfeld's help, many observers wonder why the Justice Department decided to arrest and prosecute him. Many critics believe the decision to prosecute Birkenfeld, whom some consider the most important whistle-blower in years, sends the worst possible message to other financial-industry insiders who might be considering coming forward. The Government Accountability Project (GAP), a Washington watchdog organization that has extensive whistle-blower experience, says a chilling effect is already apparent: a senior executive at a European bank that offers similar U.S. tax shelters is having second thoughts about going public because of the Birkenfeld case.