Yes, ONE TIME. Nice. But if you try to do that more than once you risk buying back your BTC at Bitstamp to a price higher than 18% as soon (or late) as your money arrives. That`s all I`m saying.
I think this scenario assumes you always have a pool of fiat and are not 100% BTC. You can do the operation multiple times, just buy on bitstamp exactly the same time as you sell on Gox, every time. Then transfer the BTC to Gox, wait a month for your SEPA to show up, then repeat.
"Just wait a month"...
It takes a little bit longer than a month
And If you invest a large amount, you're always scared that Gox will scam you, aren't you?
So it's nice for a daredevil.