Linguistics 101

Geoffrey Pullum at Language Log tells it like it is:

they not only can have a morphosyntactically (and semantically) singular antecedent, but it can do so even if the gender of the referent is known, and syntactically overt

And

This use of they isn't ungrammatical, it isn't a mistake, it's a feature of ordinary English syntax that for some reason attracts the ire of particularly puristic pusillanimous pontificators, and we don't buy what they're selling.

Heh.