I guess it would come down to a matter of how heavily involved they are in the market.

Between Fannie and Freddie, they own and/or underwrite about 50% of the nations' mortgage debt, which answers the above question with a single word: very.

Were they to go under, it's not hard to imagine that the housing collapse would be virtually complete. Now, that's not to say it couldn't recover through other means, but right now, I don't think that's a scenario anybody's prepared to deal with.