Neil Macdonald at CBC News:
No, not are. Is.
As grating as it may be to those of us who still carry Strunk and White's The Elements of Style around in our heads, the singular “they” – grammatically a contradiction in terms — is breaking out of the genderless pronoun pack, loping toward general acceptance.
The singular “they” was in fact voted word of the year last January by the American Dialect Society “for its emerging use as a pronoun to refer to a known person, often as a conscious choice by a person rejecting the traditional gender binary of he and she.”
As in: “Sally looked lovely. They wore a green dress, with their hair neatly arranged in a French braid.”
“It looks like 'they' is going to take the cake,” says Sali Tagliamonte, a professor of linguistics at the University of Toronto. “That's the way language works.”
More here.