Update: The current version provides a unified interface for four sensors:
SRF04 & SRF05 Ultrasonic Ranger
Sharp GP2Y0A21YK IR Distance sensor
Sharp GP2Y0A41SK IR Distance sensor
This version no longer uses a LUT, but calculates the transfer functions of 1/D at runtime (they are nearly linear).
Inheritance diagram:
http://arduino-distance-sensor-library.googlecode.com/hg/Documentation/InheritanceDiagram-DistanceSensor_v2.png