I am with you 100% on the Mt Gox thing. At AK Coins, an online precious metals store, we use Bitstamp for our BTC exchange rates. Mt. Gox is simply not a realistic rate for merchants to use. You can't exchange USD at the Mt Gox rate, or do it in a reasonable timeframe. Furthermore, they are always having legal and technical issues, so I have stopped using them for pretty much everything.

Unless Mt. Gox spreads tighten to nearly the exact same price as Bitstamp, they will continue to lose market share and become irrelevant.

P.S. Nice arbitrage tool. Tools like this make price discovery better and end up tightening spreads naturally. Good stuff.