California Public Employees Retirement System, the nation’s largest public pension fund, has filed a lawsuit in state court in California alleging that it suffered $1 billion in losses caused by “wildly inaccurate” credit ratings from the three leading ratings agencies.
The NY Times reports that the lawsuit focuses “on a form of debt called structured investment vehicles, highly complex packages of securities made up of a variety of assets, including subprime mortgages.” Calpers alleges that by giving these packages their highest credit rating, Moody’s Investors Service, Standard & Poor’s and Fitch made negligent misrepresentations to Calpers.
Calpers further alleges that the three agencies were “actively involved” in the creation of the securitized packages to which they gave their top credit ratings. It claims that “fees received by the ratings agencies for helping to construct these packages would typically range from $300,000 to $500,000 and up to $1 million for each deal.”

Here is a list of other securities class action lawsuits against rating agencies. Mostly relate to their ratings on RMBSs.
http://www.classadvocate.com//classaction.php?id=1300