There was one time when I did look into arbitrage across different BTC exchanges, the issue I saw then (which I'm not sure still stands) is that different BTC exchanges had different processing fees that made it prohibitively expensive to arbitrage in practice. For instance, Mt.Gox or something similar had like a 0.15% cash out fee. Not to mention foreign exchange risk if you move money in and out of European exchanges vs. US exchanges.
BitInstant was created I believe to solve this problem, http://en.wikipedia.org/wiki/BitInstant ; by parking money at all of the available exchanges and charging an interest for people to access their funds for arbitrage. They were shut down due to being deemed as a money transmitter operating without license by the New York state financial regulators.
I do think there might be an opportunity for people who trade the US and European BTC market and arbitraging the USD/Euro exchange rate.
Fr example, bitcoin.de has a best bid of 443 euro (or equivalent of 594.13 USD) of almost 3 lots; bitstamp has a best ask of 8 lots at 582.43. If you parked sufficient funds at both exchanges, you can buy at market price at bitstamp 3 lots and sell at market the 3 lots to bitcoin.de, and immediately buy a hedge of USD to Euro by shorting a equivalent dollar amount of the Euro ETF, FXE. Of course, in practice this is difficult to carry out as you have to factor in slippage, commission cost and how exact your hedge is.
Anyways, that's my $0.02; gl.
