I've wondered... if the price of a BTC differs from one currency/exchange to the next, what stops you from buying at the cheap and selling at the expensive one?
I used to do this back in early 2012. I stopped because the spread became lower than the cost of trading + forex fees and because I got nervous when MtGox took 4 weeks to wire me nearly $10,000 that I couldn't really afford to lose.
Counterparty risk is huge in the BTC world unfortunately.
I think fees on the exchanges, different currencies(fee for trading to the corresponding currency) and time(it can take weeks until you can start to trade on a site) are the major obstacles.
The time to transfer btc from one exchange to another is almost an hour. This makes arbitrage quite risky as the price is quite volatile over that time period.
The simplest version of this is to purchase bitcoins on the "cheapest" exchange, and sell them on localbitcoins for MtGox prices. Only problem here is that if you're in the US, it takes 2-5 days to get your coins, so you'd have to have a decent stash/revolving money flow.