The Sharp GP2D12 - See wiring example below.
This is the 'best' prox sensor I could find. This is an analog sensor, returning a variable resistance based on user proximity. This sensor is very useful, and does not require digital logic. See the final Butterfly documentation for examples and schematics on use.

The Sharp GP2D02 :
This is a digital sensor that returns a range of 1 - 254 based on user proximity. I used this on earlier Butterfly prototypes
(see earlier technical documentation for notes, schematics and code). This proximity sensors requires digital logic.

The Sharp GP2D05: Same as the sensor above (the GP2D02) except this sensor returns either a digital 0 or 1 based on user proximity. See earlier butterfly prototypes for examples of its use.

Simple Sharp GP2D12 wiring for the BX24:

Analog Proximity Switch with Sharp GPD12: