The last sale price determines the value of gold too, but unlike bitcoins, gold has a proven track record of several thousand years and does not rely on the Internet nor on a mass of miners to process transactions. From a wealthy investor in the USA, to a farmer in India to a factory worker in China, all would prefer a bar of gold over a USB key with a wallet.dat.
That's my point, even if all the major gold exchanges went down, people would still use it as a currency all around the world, probably silver as well. Modern currencies are all *based* on gold even though no longer backed by it. People would find it very easy to adapt to a gold economy.
If just one internet bitcoin exchange lacks enough buy orders (Mt. Gox), the coins become absolutely worthless.
Since nobody accepts BTC payments for electricity bills, wholesale goods, land, storage, it's no good as a real currency. It's only value comes from people willing to convert it to fiat money.
If bitcoin can't be converted to dollars or euros relatively fast due to a lack of buyers, it becomes dead and will hold zero value to anyone.