I agree that bitcoins need to be backed by something.
To me this means that bitcoins need to be readily convertible into the thing backing them.
Not necessarily. The idea of "backing" a currency is actually a bit of a misconception. All that is needed is to regulate the supply of the currency so that the price is at par with the value of the commodity you are using as the standard of value.
The problem I see is making it decentralized. With a single choke point a government can shut the whole thing down, which defeats the purpose. Physical metal is excellent in terms of decentralization because it's impossible to track and impractical to confiscate, but it doesn't travel over the internet very well.
I think this is possible, since we don't need any actual physical metal. All we need is the exchange rate between the metal and bitcoins. I have some ideas about how this exchange rate can be used to adjust the supply of bitcoins in a completely decentralized manner.