I think this credit crunch can be ultimately traced to the big jump in oil prices over the past year or two. Some say we've reached peak oil production globally. This probably has contributed to the weakening of the dollar, and the inability of the FED to lower interest rates now. Had oil been plentiful, there would be considerably less inflation pressure (commodities)... and the FED could have accomodated the housing bubble a good deal longer with loose credit.