Has anyone actually been asked this? When I first entered the workforce and started interviewing I remember prepping for this question (and its corresponding strengths one) and had nice prepped answers. To date no one has every asked me this - and when hiring I've never asked it.
I've been asked it maybe half of my interviews. If you're going for an interview, be ready for this one. I've had at least one interviewer be very persistent about it. There should be an equivalent question for interviewees to ask - how about "what is your company's greatest weakness?" Or "what would be the worst part about working here?"
I think the sorts of questions an interviewee asks is more revealing than any sort of digging you might do with the standard bullshit interview questions.