The Federal Reserve just concluded their scheduled meeting and announced they would purchase $400 billion in long term US Treasury notes by June 2012 and sell the equivalent amount of short term securities. The markets had generally priced this announcement in.  US Treasury prices are improved just after the announcement, particularly 30 yr bonds.  Mortgage-backed securities (MBS) issued by Fannie Mae and Freddie Mac initially were flat after the announcement but are starting to move positively now.