It isn't new, and people have been joking about finger smears since this kind of unlock mechanism came out (I remember making this discovery myself and promptly switching my phone back to using a number pad). But it is worse than shoulder-surfing; you can just steal a phone, say, from someone's purse or off a table when no one is looking, and then unlock it. You don't have to stock the person, see them unlock the phone, and then steal it.