The differences in prices are often linked to the difficulties in transfering FIAT money in and out of the exchanges.

For example, Mt. Gox is having (had?) a limitation how much they can send currency out per day - this made people buy more btc from their service rather than wait for their FIAT money to be delivered.

This resulted into Gox having bigger price in $/BTC than Bitstamp or other exchanges.

If money would move without problems between the services and their reputation are ok, people would use arbitrages very effectively.

There's always the chance of people losing trust to BTC-e, which would result into a premium price for people who dare to do business there - this is common on the markets where you can get something for cheap if you take more risk. By searching what is going on with each exchange, you will probably find relevant information why the price difference is so huge.

The point is, there's always a good reason why people are selling so much lower (or, buying higher) at one place. People don't let obvious arbitrages be there without taking advantage of them unless they have a good reason to.