It's called base money and it's backed by taxing authority. Banking is tied to government, hence the inclusion in the constitution. QE2 is legitimate, and nothing more than extra base money to replace what the market took away. We print money and give it authority by taxing power, but for some reason society cannot get over that druid metal standard thingy (which has a history of failure--ask Ben Franklin if you don't believe me).
If you don't like the Fed printing $600 bln as is necessary in the modern system then what mechanism do you propose as a viable alternate for providing a medium of exchange? I'd guess you prefer some modern wild-catting scheme, but you don't seem like the type that would allow the necessary regulation to go along with it.