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.

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.

Here is a wacky outline of an idea:  A trusted party buries an ounce of gold at a random location in the Nevada desert and records and encrypts and digitally signs the location into a token.  Then through some cryptographic magic, this information is passed along in encrypted form as the token is spent.  To redeem the digital currency for gold, some other cryptographic magic allows the token to be decrypted and there is a public registry of redeemed tokens to prevent spending a redeemed token as if it were not redeemed.  And as with bitcoin, also a registry of transfers to prevent multiple spending.

You would have to trust the initial burying/certifying party, just as you would have to trust the backing account holder or the reserve against which a note is written.  But if they are out of the loop when the redemption occurs then it is not feasible for a government to kill the currency by holding the backing hostage.

I don't know if cryptographic protocols exist that would enable the magic as I outlined it.  Maybe we'll have to invent one.