As of 6/19/2013, it has $1.9T treasuries and $1.2T MBS.
Bill Gross's total return fund returned -5.5% between 6/1 and 6/24. If this this a good proxy of Fed's balance sheet, Fed should have lost $3T * 5.5% = $165B in these three weeks, if Fed would do mark-to-market on its holdings.