It took a full explanation today from fjm to learn otherwise. "Yank" is, apparently, a specific term used in the UK to refer to a specific kind of American: rich, assertive, ignorant. It's an insult, not just a geographic descriptor. And now I can't think as to whether or not Americans ever use "Yank". Do we? Do we use it interchangably with Yankee, or do we not use it at all? I no longer know.
Since I'm now feeling cautious, is there a similar meaning divide between "Brit" and "British" when it comes to human beings?