(This is leaving aside the sociological criticisms of IQ. I don't especially want to have that debate, unless you really want to. I'm on the pro-IQ side of the debate, FWIW. But this post is mainly about the practice of measuring it, and its distribution, rather than its meaning or validity.)
I was just thinking about it because my mum got in touch with me to say she was watching a TV program where they seemed to be claiming a child had the highest ever IQ, and she said she thought mine was higher than that, and I said that yes it was (and dug the certificate out of the loft just to be sure), and that I'd known other people with IQs higher than that too, and that I'd read about people on the internet with even higher ones. I think mine is probably only average for the social group I'm now a part of. But if it's not customary among very intelligent people to get it tested, the statistics at the top end (which is necessarily a narrow tail) might be distorted due to measurement bias, and the famous very-high-IQ people (like Marilyn vos Savant, etc) might not be as much outliers as is currently thought.
I'm wondering whether my impression that "it's not customary among very intelligent people to get it tested" is accurate. You lot are part of the social group in which I consider myself intellectually average; did you ever have yours tested?