A recent review of the American mortgage crisis, Fannie Mae and Freddie Mac | End of illusions, is the best I've seen. Of course it would have been even more impressive had they pointed out the structural problems a year or two ago!
With our newly enhanced vision, Fannie and Freddie look like a classic Ponzi scheme, effectively able to issue their own debt. Their ultimate downfall came when they figured out how to evade the last vestiges of old regulation by investing in mortgages they themselves could not hold.
The emerging consensus of the economists I read is that the financial markets are now in the biggest mess since 1932, however the rest of the economy is not expected to relive the great depression. On the other hand, the Economist article ends with a curious note:
... Perhaps it is no surprise that traders in the credit-default swaps market have recently made bets on the unthinkable: that America may default on its debt.