I should add that the price of oil-denominated bitcoin, gold-denominated bitcoins, etc would have to be allowed to fluctuate somewhat separately from the official exchange rate. That way, bitcoin traders of those commodities could affect the real-life prices, and vice versa.
User A buys a USDCoin at current market rate, say 1/14 BTC. Current market rate moves to $15/BTC and user trades in his USDCoin for 1/15 BTC. Fantastic. User B decides $/BTC will move down, so he buys a USDCoin for 1/15 BTC. He is right and wants to cash in his USDCoin. Whoops! Either the value of the bitcoins in escrow has to somehow fluctuate with the asset (impossible), or you have to pray it doesn't move against the hedge.