Of course thats probably a problem that doesnt occur often but the longer it takes to repeat such a cycle the higher is the risk. I exchange bitcoins for fiat too but i do it at bitstamp because when i do it at mtgox i dont know when i get the fiat and if i get it bitcoins might be way more worth. You might say you invested USD and now have more USD. But the long time to get the USD is a risk that easily can make you regret why you didnt keep Bitcoins instead.
Arbitrage with MtGox was done pretty often some years? ago but as long as i know it was not popular then because Mtgox has these problems with paying out fiat.
I think that you see the trade this way (correct me if I'm wrong): I am buying BTC at bitstamp, selling it at mtgox and waiting for the fiat to return before buying BTC again, and thereby risking the increase in BTC while waiting on my fiat. This would be a high risk endeavor and would've caused me to lose money. This is not what my transaction entailed.