Meanwhile, many great writers — Byron, Austen, Thackeray, Eliot, Dickens, Trollope and more — continued to use they and company as singulars, never mind the grammarians. In fact, so many people now use they in the old singular way that dictionaries and usage guides are taking a critical look at the prohibition against it. R. W. Burchfield, editor of The New Fowler’s Modern English Usage, has written that it’s only a matter of time before this practice becomes standard English: “The process now seems irreversible.” Merriam-Webster’s Collegiate Dictionary (11th ed.) already finds the singular they acceptable “even in literary and formal contexts,” but the Usage Panel of The American Heritage Dictionary of the English Language (4th ed.) isn’t there yet.
—nytimes.com, 2009