"Since the Big Banks aren't taking the risks on their own balance sheets, why would anybody believe they have enough money for the back up financing, and that if they did, they would fork it over?"
Don't put all of the banks in the same bucket. Some of them have done far less subprime than others. At a few, it's virtually no subprime at all. Some banks specialize in mortgages. Others do quite a bit of other stuff. It pains me to say it, but unsecured credit card lending is actually holding up better than most mortgage operations. Much commercial banking is just fine. Most municipal business is good, at least for now.
I never understood why anyone would want to join the SIV superfund and increase their exposure, unless they were being paid one hell of a lot of money to do it.