Me too. First at the weekend (can't remember exactly what I clicked on, but I was logged in). Recognised it for what it was, ctrl+alt+escaped out of it. Ran full scan on computer - nothing.
All fine until 10 minutes ago - not logged in, it popped up when I clicked on a thread in the BA forum from the front page. I'm using IE and Vista. Came here to report it, and found this thread...
Often browse FT on iPhone, and have not had anything similar. Nothing similar on other sites.