More and more people are switching to bitstamp, and with more money flowing into bitstamp (buyers) the price goes up. Also people have probably figured out a way to arbitrage, which evens out the prices among the exchanges.
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I thought that it is still very hard to withdraw Fiat from mtgox, which should both make arbitrage impossible, and keep the price of bitcoins higher there. Has something changed? Are the Japanese able to get Yen out of mtgox?? If that happened, it would be very interesting!
There are a few people who managed to get euros out of gox in less than one week time.
Also people who are waiting for 1 month and not getting any feedback.
But it's a free market , even if there weren't problems for any of those exchanges , price would still fluctuate between them