Karen Thomson, a Sanskritist and antiquarian bookseller living in Oxford, wrote to me to point out the following very significant example of singular they in a Financial Times interview with TV writer and director Jill Soloway:
People will recognise that just because somebody is masculine, it doesn't mean they have a penis. Just because somebody's feminine, it doesn't mean they have a vagina. That's going to be the evolution over the next five years.
You see what makes this not just a dramatic claim in terms of sexual politics but a linguistically very revealing example?
The striking thing about it is that singular they is used not just in a context where no gender choice is motivated because there is no specific referent to a person of a particular sex (as in Soloway's earlier remark, "Everybody's relationship to who they are gets refracted through this gender lens"), but in a context where the appropriate gender choice could be regarded as obvious because the sex of the person referred to by the antecedent is actually entailed.
In the case I wrote about in this post (21 October 2004) the presumption of masculine sex for the referent of they was probabilistically overwhelming, not actually asserted or entailed. Thomson's case is more like this one, because we have an entailment of masculinity: the phrase "is masculine" entails the very thing that guarantees the appropriateness of the masculine gender pronoun for its subject, and the occurrence of they refers back to that subject.
If someone said Just because somebody is good at physics, it doesn't mean he knows nothing about literature, it could be legitimately objected that the choice of he for the personal pronoun prejudicially assumed that a person who is good at physics has to be male. But there's no such unwarranted presumption of masculinity in Just because somebody is masculine, it doesn't mean he has a penis: the predicate is masculine in the subordinate clause after because directly predicates masculinity (whereas the predicate is good at physics does not).
And likewise, the predicate is feminine guarantees that a feminine gender pronoun would have been appropriate; so without any hint of oddness Soloway could have said Just because somebody's feminine, it doesn't mean she has a vagina.
[Update, Dec. 1: People have begun to send me emails suggesting that I'm oblivious to the subtleties of gender identity, biological sex, and so on. I'm not. But all I need to make my very simple point is this: Soloway herself is prepared to use the term "masculine" of the arbitrary someone she is talking about, and that is surely enough to make it legitimate for her to use the pronoun he of that someone. Because if your own explicit description of someone as masculine is not sufficient to license your own use of the masculine pronoun when referring to that person, then it is hard to see that anything ever could.
The lesson is that using singular they is now so natural for speakers below pensionable age (Soloway is 50) that it is not limited to cases where a gender for a 3rd-person pronoun would be hard to assign in a non-arbitrary way (cases like Nobody ever thinks it will happen to them). The mere fact that the sex of the referent can be inferred or is directly entailed in the context is not sufficient to force the choice of he or she; singular they is available as an alternative, and for many speakers a preferred alternative.
[Further update, Dec. 2: Robert Coren has pointed out to me that Soloway might have intended her terms "masculine" and "feminine" in the informal everyday senses "mannish" (i.e. "kind of like a man") and "womanish" (i.e. "kind of like a woman"). People say things like I've always found her rather masculine or He's quite feminine in his movements. If that's all she meant (which would be much less radical: it would amount to saying that a woman can be masculine in appearance or attitude and a man can have feminine attributes, which nobody really doubts), then in fact she could just as well have used the opposite pronouns. That is, if somebody is masculine means "somebody seems rather like a man", which implicates that the person concerned isn't a man, then Soloway could have said Just because somebody is masculine, it doesn't mean she has a penis. So my main point is unaffected: Soloway could have used a gendered pronoun, but didn't.