In a few days time, US Fed will need to announce what to do after Op Twist stopped in Dec 2012 .... market basically expects further expansion of balance sheet by 40b per month (with existing purchase of MBS at 45b per month) ... so comes Jan 2013, US Fed balance sheet will probably expand at 85b per month, to give an idea, it is kind of printing 200 units of recent Prince Charles Crescent top bid per month
Sooner or later, US Fed will hold all MBS and long term UST
We are living in ever interesting world