Collateral Damage From a US Housing Bust (PDF): "After the U.S. stock market bubble burst in 2000, there were massive corporate bankruptcies. U.S. banks were clever enough to offload a lot of their commercial credit risk to other parties before the bankruptcies occurred. Thus, the U.S. banking system was bloodied by the bursting of the late 1990s stock market, but unbowed. Banks were willing to able to keep lending – if not to corporations, then to households with their homes as collateral. All of which brings me to where I came in. If the U.S. housing market goes bust, the U.S. banking system is likely to fall on very hard times as the value of all that housing collateral drops. "—Northern Trust

The report has some simple graphs that illustrate the magnitude of the danger.