Cute and quick!
Since no text is displayed, for most contacts, I only have the default avatar-icon. So it alternates two gray-scales but that does not tell me the contacts.
For contacts with no avatar or picture, could have or text (may be option with pic as well!) or a way to choose one of a few icons. Of course, I should just take some pictures and use them.
BTW, one of Nokia's weird half-touch GUIs used such a scheme for its contacts.