It's true that Gox$ aren't US$. But that doesn't mean they're entirely fake. The matter of the fact is that, some people are doing arbitrage between Gox and ot

It's true that Gox$ aren't US$. But that doesn't mean they're entirely fake. The matter of the fact is that, some people are doing arbitrage between Gox and other exchanges. And this arbitrage keeps prices somewhat in line. If the arbitrage would not exist, then the prices would be all over the place and not correlate at all. The question then is, if the Gox$ are all fake and nobody can get any, how come the arbitrageurs can keep in business? And the answer is: they couldn't. Some people get to convert their Gox$ to US$ and then use them to buy bitcoin on other exchanges to sell them on Gox. But, not all people, for whatever reason, get to do that. And the process might be lengthy, and there's a risk that it might fail for you. So the spread between Gox and other exchanges is the arbitrageurs estimate of the risk of selling BTC to Gox$ and be able to convert them in a suitable timeframe to US$.