Ratings are a factor in financial-industry regulations and in internal policies at financial institutions. But almost all of these regulations and policies treat a AAA rating the same as a AA rating. So a downgrade from AAA to AA would trigger little or no forced selling, Arora said. An official at Vanguard, the big mutual fund company, told me this week that "a downgrade from AAA would not trigger any events inside our funds."
From Planet Money's blog, http://www.npr.org/blogs/money/2011/07/27/138738198/3-reason...
The full podcast, "Would a downgrade matter?", is an excellent listen: http://www.npr.org/blogs/money/2011/07/28/138721364/the-tues...