That actually is grammatically incorrect in English even though it is used often in common parlance; they is supposed to only be plural. The correct usage would be "he or she" if you don't know the gender of someone; the reason why "they" is used so commonly for singular is because of how unwieldy it is to say "So about your friend; does he or she bring his or her ipod on his or her way to work?"

It's actually a huge flaw in English, and there has been tons of proposals of gender-indifferent singular pronouns like "xe". On the other hand, "you" used to only be plural and apply if you were talking to a group of people ("thee" was singular) and obviously the correct term today is you for both singular and plural.

It's easily possible that within the next 50 years it will be commonly accepted to use "they" as a singular pronoun in formal communication, but its not today. You would almost certainly be marked down for using it in that manner on the SAT or GRE for example.