But it seems as if the difference between Mt. Gox & Bitstamp price serves as a indicator for the future price of Bitcoin. I can't really get any great chart of this, so its hard to visualize, but I've been looking at it and it sure seems like it at the very least confirms the trend.
When the potential for arbitrage across these two exchanges is high, then the price drops. Sooner or later the Bitstamp price starts climbing and the Mt Gox price stops falling, and the spread between the two starts getting smaller and smaller. Indicator to BUY.
During the rally that took us up to 136 there was also a big gap between Bitstamp and MtGox, so I don't think this is a very good indicator.
Note that gox has lost a lot of market share to bitstamp since then. Back then since Gox is generally higher price than Bistamp, if Bistamp followed Gox, it would make sense that the indicator wold have been inverted.