Haptic feedback will be a great addition to the next iPhones (and hopefully iPod Touches!), and it paired with fingerprint recognition seems like a perfect match. However, I just hope Apple doesn't overdo it. Memorizing all these gestures might become a pain, depending on which gesture it is. n degrees within the reference axis? But I can see it being used in very interesting ways.
I'm worried about battery life a little, though. Even with the (slightly) improved battery life of the 3GS, thousands of tiny pizoelectronic actuators vibrating at all sorts of frequencies might wear on the next iPhone's battery pretty quickly. Also, assuming that the pizoelectronic actuators are below the surface of the screen, this could signal OLED screens in the next iPhones. I know almost nothing about the technology, but its nature of being basically "printed" onto a surface means that the screen can be much thinner, an advantage if extra thickness is required for the sensors.