The only real arbitrage I did was with bitcoinbuilder. I deposited money into Gox and got it out with around 70% profit margin. I was lucky to get it out in time, because two days later they froze their website. I don't see these opportunities anymore.

If you do arbitrage now, then if you calculate the arbitrage opportunities between exchanges after transaction costs, you can get on average 0.10$ per trade. So your just scalping the markets. This small profit margin wipes out very fast when you make some mistakes or do the wrong trade.  Also you have to do alot and very fast to make it profitable.