Ask HN: Why doesn't arbitrage appear to equalize the bitcoin market?

At this time, Bitstamp lists btc at $102.98 usd, and MtGox lists it at $118.88220 usd. Why are the markets so inefficient that the gap is so large? Is it a volume issue? Bitstamp lists the 24h volume at 20459 BTC, and MtGox at 53187 BTC, so between the two there's about 9 million dollars of btc trading hands per day.

Is it that the potential profit at the current volume levels is too low, or the potential profit at the currently imposed marketplace limits is too low, or is it that dollars-to-bitcoin transactions take too long to complete to successfully cash in on the difference, or is it something else?

I'm very interested in bitcoin and would be fascinated to hear any theories/explanations.