To kick it off, a suggestion from a Yank that maybe people are just being polite:
>>I've been following the Walk Like an American series with some interest, but there's one aspect I don't think has been brought up enough. Given Americans', ahem, reputation, it might be common that when not exactly sure where someone is from, tell them you thought they were from somewhere else. Americans won't mind, but other nationalities might be offended to be mistaken for us. I've had a friend tell me that when I think an accent is Argentinean, ask them if they are from Uruguay because an Argentinean will think this is amusing and well intentioned while a Uruguayan wouldn't be as happy about being mistaken from an Argentinean.More below.
Now, in my experience people wouldn't really care that much either way, but I can see the logic about going out of your way to avoid assuming someone is an "undesirable" nationality.<<