Except arbitration requires more than just money + information, people doing arbitration must have low fees / latency for example so it's not available to all players in the market. Net result real world markets are not efficient due to various access levels.

Also: Empirical analyses have consistently found problems with the efficient-market hypothesis, the most consistent being that stocks with low price to earnings (and similarly, low price to cash-flow or book value) outperform other stocks. Which is presumably due to cognitive bias. http://en.wikipedia.org/wiki/Efficient-market_hypothesis

PS: It's a reasonable simplification that's useful for the average investor, but not policy makers for example.