Guess you don't count preferred since that is an "Investment" in the company and not an obligation. I do not believe any of the junior or senior preferred is "callable" so they NEVER need to pay it back. Thus it won't count against the net worth.
However until the seniors are paid back they can't come out of conservatorship or pay dividends to any other stock... But there are lots of great companies that never pay dividends so... no big deal.
It will be interesting to see how the market prices Fannie when they report a net worth of $10B in November... I guess common stock will be $10.