This is nothing new. I remember an article/paper about the same thing but with pin pads.

Keep in mind, it only works for a very short amount of time as the surface cools down quickly. I don't think it's an attack vector you should worry about.

You can just as easily shoulder-surf the user as he unlocks the phone. It works even better with the "stroke gestures" that is common on android devices - most users don't disable the "draw line" option... it's much easier to spot and remember than a pin pad entry :D