Vitus has discussed the "Banks not lending" issue before (eg, 3/4/10). It's nice to see, 2 1/2 years later, people making the same arguments.
Proprietary trading source: Banks, who are keeping credit conditions tight, have to manage certain types of risk when taking on a fixed rate loan, such as interest rate risk and credit risk. Because the cost of credit remains at 0, and loans are at say 3%, there is the risk that rates could rise and these loans become unprofitable. With the Fed extending their interest rate guidance out further and further, it is in effect reassuring banks that there is no rate risk for the foreseeable future. The other part, of course, is that it is making the safety (Treasury/MBS) and risky asset (corporate) trade unprofitable for banks, forcing them to lend to consumers. Bernanke is a big proponent of not repeating the mistakes of the Great Depression where the Fed tightened too early and slowed the flow of credit. A consumer needs credit like a human needs oxygen, so his attempt is to keep the nozzle as wide open as possible.