Bad idea; its best to just allow the worth of bitcoins to be based on the faith of the consumers & service providers - that's pretty much what any currency is based on. As someone already pointed out, gold is physical and therefore controllable by those who have it. They'd become like puppetmasters of the bitcoin market.
Edit: If you think about it, the silver that goes into making those high performance mining rigs kind-of already ties silver to the system via MINING, but since difficulty goes up over time that becomes less and less valuable.