Warren Buffett has been taking some heat lately because he is the largest shareholder in Moody's and Moody's is one of the government approved rating agencies who greatly enabled the recent financial crisis with absurdly generous ratings for mortgage backed securities. While I don't think this has been Buffett's finest hour, I think some the criticism has been a bit overblown. Expecting people to be objective about matters in which they have a large financial interest is about as realistic as expecting parents to be objective about their children. As Buffett himself has said, "Don’t ask the barber whether you need a haircut.".
For my part I think the current system provides unacceptable incentives for corrupt ratings and needs to be drastically reformed. New York State requires me to get my car inspected every year. It does not allow me (as far as I know) to go to several inspectors and only pay the one who agrees to pass the car. Nor does it allow me while doing this to swap out all the parts in my car until I find the minimum configuration that will pass. It should be obvious that allowing this sort of thing is a bad idea but that is how the security rating system currently works.