Here's a topical quote from Roger Lowenstein's excellent book 'The End of Wall Street'.
'When Lehman took a mortgage pool to Moody's, it paid a fee only if it was pleased with the rating. Otherwise, it could try its luck with S&P or Fitch. To understand the threat to the integrity of the process, imagine the big rating agencies as three competitive saloons standing side by side, with each free to set its own drinking age. Before long, nine-year olds would be downing bourbon'.