As I understand it, exchanges where it's hard to get your money out as USD (e.g. Mt. Gox) have high USD exchange rates, whereas places where it's easier (e.g. B

As I understand it, exchanges where it's hard to get your money out as USD (e.g. Mt. Gox) have high USD exchange rates, whereas places where it's easier (e.g. BTC-E) have lower rates. In theory if the exchanges were all equivalent in this regard then the rates would pretty quickly stabilize, because people would use USD to buy BTC low on BTC-E, send it (as BTC / for almost free) to Mt. Gox, then sell the BTC high on Mt. Gox for way more USD, and repeat and profit until the rates converge. (see "arbitrage") But it's not easy to get USD out of Mt. Gox last I checked, so that doesn't seem to happen.