Ultrasonic Sensor's family tree...

I'll file this under "Interesting History Lesson" - John Barnes (HiTechnic) has been working on his version of the ultrasonic sensor for a while... check out this article on one of his early designs... and scroll down to see an early version of the NXT's USonic sensor... love the 'eyes' on it...

(Notice the date on the circuitry schematic)