Good luck to everyone arbitraging BTC --> MtGox JPY --> JPY --> USD --> Bitstamp USD --> BTC !
If you are successful and report it here it will likely decrease arbitrage opportunity since it would change market confidence. It's possible that psychological effect would have as great an effect in reducing gap as increasing bitcoin supply in JPY market. Thus I don't expect to hear many positive results other than people who do an experiment for fun. Nevertheless I will be happy to be wrong!
Interestingly it might be possible to distinguish the supply and market perception effects.
Arbitrage should bring the Mtgox JPY price to parity with Bitstamp USD given USD/JPY exchange rate. Then the change in the gap with MtGox USD / Bitstamp USD between now and when JPY exchange parity is achieved will show the psychological shift since a reasonable hypothesis is that most people who are causing the gap on MtGox USD / Bitstamp USD (by buying at premium as opposed to waiting) do not have the possibility to exit via MtGox JPY --> JPY --> currency exchange --> USD.
Q. Do you keep independent ask/bid tables for different currencies?
"A. No, ask/bid tables for different currencies are not independent. All currencies are relative to whichever currency has the highest volume, which is based on said currencies current market price in bitcoin. Every trade is in one pool and in fact, not are not separate currency markets. This allows users the added benefit of trading in "the greater market" -in currencies they understand- while not limiting them to smaller currency markets."
https://support.mtgox.com/entries/20800336-Multi-Currency-Trading