The Volokh Conspiracy has a two posts (here and here) on the distinction between gender and sex. He had used "gender" to refer to a person and got an annoyed email saying "gender" refers to words while "sex" refers to people. The second post says:

Several readers pointed out that in some academic circles, sex and gender have acquired specialized meanings, with sex meaning the biological attributes of sex, and gender meaning the socially constructed ones. That's a point worth remembering for technical circles; but my experience is that this distinction has not dramatically influenced lay usage, and my suspicion is that it probably won't, at least for quite a while.

Now it seems to me that in the circles which I run, this is the standard lay distinction, at least if you think about it at all. That is to say, gender is the prefered term unless you are specifically talking about biology. I even checked asking one of my friends from home to define the difference cold and he said: "my personal distinction would be that sex is the physical aspect.. but gender is more of the mental alignment." Anyway I think this is entering lay usage at least in certain circles an awful lot faster than Eugene Volokh expects. What do you guys think? One other theory is that people use gender almost exclusively so as to avoid double entendres.