Sorry to butt in (this conversation has been fascinating by the way) but would IQ tests be less relevant as you get older? If you take the tests frequently, i would have thought that your mind would "adapt" to the style of questions (for instance pattern recognition) and so the actual result would be more deceptive as time goes on.
I think if i was a member of MENSA or NAGTY (to be honest my personal opinion is that i don't have to be a member of a society to be or show taht i'm intelligent) i wouldn't put it on a PS. I would put it on a CV or if there was another section to an application form such as, oh i don't know, other interests. But as the general consensus seems to have shown, i doubt there would be much point in putting it on a PS if you haven't got something subject relevant out of it. I think my other opinion would be that it would take more than a sentence to say that you were a member and that you had been to summer schools, etc, it would need more justification. For instance, i went to CERN with my physics department early this year and it's taken up more than one sentence for sure - having first stated the fact and then gone into slightly more detail about what we actually did there, relevance to my Physics education, etcetera. Whether this is just my style of writing (or whatever you want to call it) it would take me more than one sentence, which i could have put to better use.
(sorry if that rambled a bit :P)