An arbitrage trader follows the same "buy low, sell high" strategy of an ordinary speculator, except that he doesn't have to guess the future -- he can do both trades at the same time.

I imagine that arbitrage should be particularly lucrative for exchange owners, especially if they collude.  

Suppose that owners Alice and Bob of exchanges A and B agree to show each other the first few entries of their order books in real time, while delaying that information to the public by a few seconds.  Whenever Alice sees a sell order in her exchange A that has a lower price than the highest bid in B, she immediately buys into that order, counting that Bob will immediately sell the same amount into that bid. And vice-versa.

If this scenario is possible, the partnership Alice&Bob obviously would make a profit on every such occasion. Alice and Bob could equalize their expenses and profits, at any later time, just by exchanging bitcoins. Thus, even if A is in Bulgaria and trades only with dollars, while B is in China and trades only with CNY, they would never need to exchange dollars to/from CNY.  The dollars that Alice would take home would have come from the dollars that her clients deposited in A, while the CNY that Bob would withdraw from B would come from the CNY that others deposited into B.

(Trading fees, bank fees, and currency exchange rates complicate the details, but do not seem to invalidate the idea.)