When I was in Germany for a semester, people always thought I was French. Granted, I was near fluent in French at the time, so I played along, but it was rather disconcerting the first few times, and then when I was in Paris and was stopped by an (obvious) tourist who asked where the Metro was! Feeling slightly wicked, the instructions were given in French, with my rather Paresian accent. The only times that I was pegged as American was when I was with my classmates, who did fit the stereotypes of "loud, slightly rude, partying college kids."
My background, however, is British/German/Polish.