There is a paper at WOOT 10’ described how to use smudges on the touch sceen of a smartphone to get largely decrease the time an attacker need to guess the right password to unlock the screen. For example, by for 4 passcode based iPhone, one just need to try at most P(4,4) = 4! = 24 times before he get the right one.
But I think this situation had happened on PC and we already have a solution.