Peter Seibel--"either a writer turned programmer or programmer turned writer"--considers pronoun usage at A Billion Monkeys Can't Be Wrong:
For better or worse, readers can be thrown for a loop by clumsy diction, abstruse vocabulary, or violation of what they consider norms, whether grammatical or social. The harsh reality of a writer’s life is that some readers will be jolted out of their concentration by a singular they while others, who would have glided right past it, will trip over a gender-neutral he.1 There’s no good way out of this mess, at least in the short term. ... On the other hand, I’m quite certain that singular they will prevail in the long run. It was standard usage long before the 18th-century grammarians put their oar in and it is even more attractive now for people who wish to avoid implying that executives and grammarians are always men. It also follows the pattern set when the plural pronoun you drove out the singular thou.
1. And some of us will be distracted by both, noting both singular theys and gender-neutral hes as instances of a writerly choice.
I recommend reading the entire post, which is thoughtful, logical, and well written.
(Via Language Log.)