Quote Originally Posted by Оля
But "Вы не знаете Ивана Петрова?" means only "Don't you know Ivan Petrov?", that is just means the question itself.
I understood the question as a polite variant of "Вы знаете Ивана Петрова?". Similar to "Не знаете ли вы Ивана Петрова?".

Thus we have agreed about difference between polite and literal negative questions, but put this question into different categories.