This is true; not a word of a lie. I once held a door open for someone in Erie, Pennsylvania, and she said, "Oh, you must be Canadian!"
There are worse national reputations, to be sure. But I mean, seriously, are you telling me it's so unusual to hold a door open for someone in the United States that your conclusion isn't, "Wow, what a nice person," but, "Wow, this person's from another country..."?
What's the thought process here? "Whoops, this person's being considerate. No American would do that. He must be a foreigner. But from what country? Well, given the geography, the most likely one is Canada. Let's check that. ...Hmm, yes, it says here that Canadians have a reputation for being polite. All indications are that this person must be Canadian. In fact, I'm so sure of that conclusion that I feel justified in saying so out loud!"
How could she lose? Either she's on the money, in which case she's going to flatter a Canadian, or else she's giving a sort of complement to a fellow American on the basis of a shared cultural stereotype... still a winning move. There's an off chance she'll actually offend a fellow American, who might respond with, "What, you don't think an American can be polite?" But let's face it: if he's polite enough to hold the door open, he's probably not rude enough to say that. So, like I said, a winning move all around.
But I think all Americans should do this. Regardless of whether they think the person they're talking to is actually Canadian or not, whenever someone is even the slightest bit polite, they should say, "You must be Canadian!" I think this would promote an ever-increasing level of politeness building on politeness, until at last, the US was indistinguishable from Canada in this regard.
Then, we could take over. Everything from Galveston to Alert would be one place. And at long last, that greatest of dreams would come true...
We'd finally have Alaska. :)