It's one of those questions that provoke the most thought, allow granular benchmarking of the interviewee's knowledge, and have a valid answer, in my opinion.

That is to say, this question has worked for us as a dev house, maybe not so much if you're hiring contractors, I can't say for sure.

That sounds more like a great interview question than a good FizzBuzz question though.

FizzBuzz questions are all about assessing, in less than 30 seconds, if you should even be giving the person on the phone the time of day.