Feature request: calculate a weighted average of and/or history of trading prices for each fiat currency.

I know that most people price their trades based on some formula using bid/ask/last prices from the major exchanges.  However, given the divergence in prices between Mt. Gox and other exchanges, it would be very useful to have a bid/ask/last/depth based on what local bitcoin traders have actually decided to set their prices to.

As the volume of local exchanges increases, this consensus could displace using unreliable centralized exchanges as a pricing mechanism, and instead show the market price when taking into account things like the risk premium of in-person trades and the delay times involved.

Another interesting set of data would be to plot the market price in different geographical areas vs. the average for that currency.

I'd contribute some BTC to a bounty for implementing this.