UPDATE: to post 14th July 2008

Fannie/ Freddie collectively have $5,300 billion in mortgage credits against $ 81 bn in capital.
Any revaluation of their mortgages by more than 1.5% discount will wipe out their capital base.

More than half their loans are from the peak years of 2005 – 2007.
Valuation of their mortgages is difficult, but it is certain they are
worth something less than face value. While they had a rule of not lending greater than 80% of home value, this rule was broken through top up loans, and potentially through loans purchased from investment dealers.

Fannie and Freddie double the US national debt « The Bankwatch

Nonetheless its no small leap to realise that the US government is now on the hook until the underlying asset value is clarified.

Reference ft.com