When do Fannie and Freddie consolidate?

A question to anyone who knows _a lot_ about public sector accounting. Just how much help exactly can the U.S. provide to Fannie and Freddie before those $5T of mortgages consolidate onto the government balance sheet? And would the US still be AAA with another $5T of liabilities?

Update. Bloomberg has caught up with this one.

There's nothing sacrosanct about the U.S.'s AAA rating, no matter what dogma and orthodoxy might suggest. Many financial assets that claimed AAA status before the credit crunch turned out to be irredeemably tarnished; there's a non-negligible risk that Treasuries will prove to be similarly spoiled.

The ten year CDS spread on US treasuries settled in Euros is now more than twenty basis points.