A very good discussion on this topic ensued from a very general question.  One responder got the most important part of the formula - i.e. that you need equal values on each exchange so that you can go long/short simultaneously.  However, you still need a way to "return" funds to the starting point in order to repeat - this is typically done at a loss, which obviously needs to be less than your gain.  Another important consideration is that you will need to hold a certain value of crypto currencies at all times - your investment in those holdings will be subject to market price fluctuations - this is not necessarily bad, but it is a risk you need to be aware of.  In general though I would not recommend doing this without a well thought-through strategy and the ability to take a risk on a portion of your investment.  Lastly, be aware that the exchanges are not as secure as banks.  BTCE is a recent example of this peril.