Given the incidence of IQ 180, or even IQ 140, people in the general population - at least according to the various charts that I have found via Google and that others have referred to - there seems to be very little danger of this.
I find that argument to be a bit hollow. Arguing that it should be fine for a 5 INT character to be played as a super genius because the distribution of actual super geniuses is so low that it would be have a vanishingly low incidence of occurrence isn't convincing. You're declaring that it's okay AND a problem, but that it's chances of occurring are so low that that problem part isn't likely so it's really just okay to do it. Can't agree with your conclusion, there, even as I agree with the statements about likelihood. If it suits you, how about a 5 INT played as a very bright person, but not quite a genius, or whatever preferred vernacular you have in mind.
Also, I am curious as to what you and others think should be the penalty or consequence for a player who plays his/her 18 INT wizard as if s/he had only a normal intelligence - which surely must be the case for the vast number of 18 INT wizards, given that (by definition) most players have more-or-less average intelligence.
Just as I don't expect a 5 INT character's player to actually have a 5 INT, I don't expect a 18 INT character's player to have an 18 INT (whatever you decide that means, I really couldn't care less about the argument relating to equivalent IQs). In that case, if they're they remind me occasionally that they're playing a genius and ask what that gets them, I'd be glad to help out. On the flip side, if they were playing a low INT character and did similar -- ie, occasionally pointed out that their not bright and how that might affect them -- I'd do the same, help out. Or, more likely, let other players chime in. We typically model high intelligence at the table by allowing the group to discuss and plan collectively and then the high INT character takes the in game credit for the plan if it's necessary, say in presenting it to other allies. No one's requiring people to be actually smart or actually dumb to play their stats, it seems to me people are just asking for a good faith attempt to do your best at modelling your stats in an appropriate manner. I'll admit that I don't have a good definition of appropriate, it's like porn, I know it when I see it, but I find that a bit of discussion, an open mind, and referring to what the rules suggest are good starting points. If you have a creative way to roleplay your stats, high or low, and me as the DM and the group as a whole are good with it, let's roll. I'm pretty open about concepts and fluff, I like creativity and try to reward it. On the flip side, though, I dislike attempts to powergame around disadvantages by making them irrelevant.
As an example of that, much earlier in the thread Iserth had an example of playing a 5 INT character as a Sherlockian sleuth. I'm fine with that, so long as the player provided, for example, that this would be a delusion of the character, who isn't actually capable of Sherlockian deductions except rarely, and we worked in an appropriate check. Others may be initially impressed by the sleuth and his quick pronunciations of solutions, but would rapidly lose faith in the character's abilities after a number of likely failures (or not, who knows, maybe the dice were hot that night). I can work with that. What made me upset at Iserth's example, though, was that he said that if he roleplayed the sleuth as smart all night and never had to make a check, that he'd be good not ever telling the DM his INT was a 5. To me, that's bordering on outright dishonesty and cheating. Not the roleplay, but the intentional hiding of a disadvantage relevant to the play at hand. If I had a player do that in my games, they would likely be disinvited barring some other circumstance.
