Bitstamp allows deposits/withdrawals using SEPA (EUR only), but all traidings are done using USD. It is nowhere described what exchange rate is used when doing these deposits/withdrawals.
There are some indications that amount of money lost in conversion may be significant:
- The order book only offers bitcoin-dollar trading so conversion fees apply on deposits and withdrawals (the conversion spread can be queried by API). 
- Bitstamp has an EUR bank account in Slovenia to which I deposited euros, then it converted to USD. Then it converted the EUR deposit to USD but I checked the daily EURUSD forex range and Bitstamp converted my EUR to USD at a rate worse even the worst rate in the 24-hour daily range; I have to say, clever: another small profit source for them. 
Does anybody know how big the EUR/USD spread at bitstamp really is? Is it significant?