Circuit diagram of the ultrasonic sensor has been exposed as one of the "HDK" at the site of the LEGO.

This is a circuit diagram. The output of the receiver seems to amplify by using four transistors. It is a technology called I2C to communicate with the NXT. If you use the I2C, you can convey the distance as digital information to the NXT. If you want to see more detailed circuit, please download from the site's LEGO.