My idea: miners add the current exchange rate to every block. Anyone cheating will simply create orphans, so you don't have to enforce a specific quote source; each new miner just has to agree that your quote was fair +/- a little bit (just like they agree now that the transactions you signed were accurate). From there you just apply a smoothing algorithm to it (perhaps a mere SMA, but I have more sophisticated methods in mind) and adjust inflation / demurrage to compensate. I can think of similar methods to handle other aspects of monetary policy in a decentralized way, but I don't want to get too far off the subject here.
If this is debunked, can you link me to where I can read more?
What exchange rate do you use? Bitcoin to dollars? Bitcoin to euros? Bitcoin to pounds? Bitcoin to rembini? A basket of all the previous ones? Who decides if the index of the baskets need to change because the fundamental economic situation has changed (the euro has gone down, the dollar has lost reserve estatus, etc...). And then where do you get the data from? MtGox? TradeHill? Others? Combination? What happens if some of those companies goes down? Who is going to trust in a currency that needs constant changes and agreements to work? What happens if the miners dont agree? How do you deal with the uncertainty?