Anyone see this report on CNBC? It does a great job of outlining the breakdown of the mortgage financing debacle at all parts of the transaction - loan origination, mortgage bundling/securitization/sale on secondary markets, credit rating providers, end investors.
Here's a link to the article w/ times of the replay:
http://www.msnbc.msn.com/id/29163182/If investors have no appetite for mortgage backed securities in the future, the secondary market will be a fraction of what it was at its peak. And if that's the case, then banks/lenders will always have a limited ability to replenish funds to offer new mortgages to buyers.
If, going forward, the secondary market is (for all intents and purposes) gone, then lenders will have to lend from their own portfolio - which will do two things: 1. limit the supply of money for mortgages, and 2. tighten underwriting guidelines even more as lenders will now have their own funds at risk.