I always wondered whether ultrasound devices were not explored due to production/cost issues or for some other reason. Years ago (pre-iPhone) I wanted to do passive user recognition on a touchscreen music product because some Polish researchers showed it was possible to image fingerprints well enough for recognition with ultrasound sensors placed around the bezel. Why carry RFID or depend on BT or something external when you can do it internally without any configuration? Maybe someone knows more about actual manufacturing of ultrasound tech...