Imagine that btc-e become a gateway too. Then you could immediatly transfer some USD from bitstamp to btce to do some arbitrage. You could also send your bitcoin in instant (instead of waiting +/- 45min) to btce, trade it then withdraw it.
wait does it mean that we can we actually already make such arbitrage between stamp and kraken?


Nop cause Kraken is not a gateway YET. But it will probably become one soon. (it already trades XRP, its bank is Fidor, they said I don't remember where they're aiming to become a gateway).
They said it in January this year, without giving any target date. It is somewhere on their webpage, in FAQ I think.