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I've seen it happen but it was some time ago IIRC. And it was not really enough to cover the 1% exchange fees plus whatever fees are associated with transferring a lot of money (if one needed to do that.) But I think that the best way to arbitrage would probably include some amount of picking off spikes and dips anyway.
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Just a heads-up/reminder for anyone interested in arbitrage.
Good (and unusual) opportunities to move funds the easy way with Tradehill holding notably above Mt. Gox for reasonable periods of time of late. Probably only a handful of relatively smaller players could do it though such is the depth at Tradehill.