what stops me for ex. buying here:
https://btc-e.com/ where i see the price is @40 and then transferring the coin to mtGox and reselling at 42??
That's called performing arbitrage and is a necessary component of keeping prices between exchanges in equilibrium.
The problem is in moving cash to and from the various exchanges in order to do that trade. Also a person trading on various exchanges needs to consider counterparty risk -- meaning not only must you trust that the exchange won't run off in the middle of the night with your funds, you have to trust that they are competent enough so that your funds won't be stolen and you'ld be in the same condition, without access to your funds.
Both BTC-E and MtGox have had security breaches in which customer funds were stolen. In both instances the exchange covered the losses, but in neither instance apparently was the amounts stolen above the levels that would make the exchange insolvent.
So generally, traders doing arbitrage will limit the amount of funds to a minimum level.
So right now the difference in the exchange rate between BTC-e and Mt. Gox consists of the difficulty and cost in moving USDs from a bank account into BTC-e, combined with the difficulty and cost in moving USDs out of Mt. Gox (after selling bitcoins) to a bank. In all, that's requiring a lot of USDs to be "in play", and thus exposed to exchange rate risk (can't simply buy bitcoins on demand with the funds should market conditions change).
So you are free to give it a try. Just that the nearly $2 per bitcoin difference ends up being so little profit by the time all is said and done.
