In this video from NTT in Japan, they have a heat-display on a mouse. A surface changes from hot to cold depending on the voltage applied, and the sign on the voltage.
Imagine putting this on a phone.
More immersive movies. Heat at explosions, etc.
Feel a spot on the device in your pocket. If it gets cold when you press it, the sound is off.
Send vacation messages and convey how hot/cold it is.
Weather widgets can be more intuitive if they display heat/cold.
Wikipedia describes the Peltier effect.