Based on what I've been able to discern there are two causes to the current market crisis. The first began a decade ago when our meddling politicians decided that the lenders were being racists by not lending to poorly qualified minorities in slummy urban areas (CRA). So the government relaxed lending standards and created tremendous liquidity. This allowed people to refinance property and created two bubbles - first the stock market and second in real estate.
I was able to purchase property because of this liquidity with virtually nothing but my credit score. Some of the property I got was foreclosed when the homeowners couldn't pay after borrowing many times the value of their property. (One property I got for $5k had a $71k loan against it. In truth it was probably only worth $1k, but it was owned by a minority and no one wanted to discriminate by valuing the house at its real value)
The second part of the crisis is due to a FASB accounting practice called "mark-to-market". About 8 months ago I started getting letters from all my HELOC's stating that the value of my property had fallen and my credit line was no longer available. Although property values in our area have increased, suddenly the loans I owed money on were valued at over $100k more than I owed on them. So the big companies like AIG and Lehman had to revalue all of the loans to market value suddenly showing tremendous losses and cutting all market liquidity. It didn't matter if I (and most other homeowners) were still credit worthy and paying off the debt. These firms found themselves stuck.
So the politicians that created this mess are all decrying the "greed" of the Wall Street managers (and no doubt there is greed there). But the truth is that Washington created this mess by trying to interfere in the market.