Corporate Fraud

The news today is about Enron's Lay and Skilling being found guilty.
There is also a side article about auditors.

MSN Corporate Fraud Alive and Well "....But those auditors — typically larger, big-name accounting firms — haven’t done such a great job, according to the Public Company Accounting Oversight Board, the body that was created by Sarbanes-Oxley to audit the auditors. After conducting inspections on those accounting firms' operations, the board found last November that three major accounting firms — Ernst & Young, Price WaterhouseCoopers and BDO Seidman — were failing “to identify and appropriately address errors” made by their clients, including mistakes that were “likely to be material to the firms’ financial statements.” The board and the firms say they are working to correct the problems."

My question then is "Who audit's the auditors?"