I don't know why anyone hasn't mentioned the hours long confirmation delays for BTC transactions. It's not possible to effectively arbitrage (without exposing yourself to risk in the form of hourly price changes) between exchanges. You can't flip BTC that fast at the moment.
You can use a faster blockchain to move between exchanges if they both support it. But there are still the inflated confirmation requirements for withdrawals, that most exchanges impose, to slow you down. Not to mention daily withdrawal limits.
