okay they deal mainly USD everything else has to convert to USD before trade?
Not everything, you can have a sell order of 1BTC for 10EUR, and someone could buy that 1BTC for 10EUR without ever converting to USD. Its just when there isn't enough market depth in the other currency, they start buying/selling into USD orders.
you bet me to it, was going to say:
they allow you to trade your EURs for Bitcoin directly, its just that the big walls on EUR MtGox are owned by MtGox, when you buy or sell into their walls, they turns around and buys or sells on the USD market.