With volatile exchange rates (both in BTC and lately in the other currencies as well) and markets in over two dozen currencies, the are plenty of opportunities for arbitrage.  The order sizes are generally small though so the amount of profit from arbitrage isn't enough, it appears, to attract automated arbitrage trading just yet.
 - http://en.bitcoin.it/wiki/Bitcoin_Arbitrage_Opportunities
 - http://www.bitcoinmoney.com/post/7000316379
 - http://www.bitcoinmoney.com/post/7259588518