Thank you very much. I was somehow unable to find this bit in the FAQ. Indeed, if Gox randomly executes multicurrency trades with 2.5% fee, that's one more reason to stay away from them....
Well I'd like to think of myself as a reliable source.
https://support.mtgox.com/entries/20800336-multi-currency-tradingQ. Do you keep independent ask/bid tables for different currencies?
A. No, ask/bid tables for different currencies are not independent. All currencies are relative to whichever currency has the highest volume, which is based on said currencies current market price in bitcoin. Every trade is in one pool and in fact, not are not separate currency markets. This allows users the added benefit of trading in "the greater market" -in currencies they understand- while not limiting them to smaller currency markets.
For example, if a buy order for bitcoins is placed in EUR, the order can be executed against another user selling bitcoins in any currency and not necessarily only against another user selling bitcoins in EUR.