A few notes on this:

1) and keyword for c++ ? It has been a while since I used C++ but it used to be &&. Did it change (I'm actually asking, not being sarcastic).

2) on the 'else if' parts, you only need to do one evaluation (the == part) since the first if tests for both % 3 and % 5 so no need to do != on % 3 or % 5 as you know it will never be true.

3) honestly, I think the comments make it worst. If a programmer doesn't know by just looking at the code what it does, no amount of comments will help him.

I don't think those comments are very helpful. I think a function-level comment would be much clearer.

  # Outputs 'Fizz' when +arg+ is divisible by 3, 'Buzz' when
  # it is divisible by 5 and 'FizzBuzz' when it is divisible
  # by both 3 and 5.

That's a matter of opinion. I always do this. Now there is no question what each conditional is intended to do. So if I make a mistake, you can clearly see from the conditional comment what I meant to do.