Your right, a robot on an assembly line will never feel pain. A robot built like a planetary lander will acknowledge damage. A robot that we've all seen in science fiction shows that behave very human like is altogether different. Today the processing requirements necessary just to stand vertically is substantial. To move, act and be able to communicate will come sooner rather than later. Of course you can program the robot to operate very simply but I remember watching a show about AI and in simple desktops, programs that are most frequently used will start up faster because the system has learned your habits. This hadn't been programed, the system learned by itself, is this a step towards sentience? Now add this to the computational resources of a robot that has the ability to walk and communicate. Will it learn, will it develop a sense of self, of emotion that will interpret the sensory messages as pain.