The NYSE processes millions of orders per second, MtGox hundreds per second. So you have machines which work over a protocol (FIX) to establish the price mechanism. In a exchange system there are limit orders and the limit orderbook. You basically have a timestamp problem, which is kind of prependicular to the bitcoin system. At every point t in time, there is a fair value of BTC-USD. And a system which solves that problem will be highly centralized, by definition. So unless you come up with a new system for processing limit orders, people will be arbitraging between a slower system and MtGox, Bitstamp, etc.