This might be a bitcoinwatch FAQ. Many people often assume that the gold/BTC value is simply calculated from the BTC/USD price, after getting a spot gold quote from some third party financial website.
The above method described by jgarzik would be the best way to show BTC/Gold in $. I don't know how difficult it is to pull Gold/USD prices from a third source, but definitely this would be very accurate.
This would also be a phenomenal measure of success for Bitcoins overall. Since its creation, BTC has outperformed Gold by galaxies.. We should track how this continues.
And then, if BTC/Gold continues to perform well, this could be used as a strong marketing sell to the public:Bitcoins is Gold 2.0 - better, faster and much lighter.