I have a semi-decent version of the generic DistanceSensor class.
(warning: the code compiles but is not yet tested using actual sensors)
Top level class: DistanceSensor
Level1 classes: hardware type/group specific code: class for analog sensors and class for timing based/acoustic sensors (AccousticDistanceSensor, AnalogDistanceSensor)
Level2 classes: sensor type specific code: (DistanceSRF04, DistanceGP2Y0A21YK)
Now adding other sensors with different LUTs or timing options should be straightforward: just add a new "level2" class.
Source: Google Code Archive - Long-term storage for Google Code Project Hosting.
Doxygen documentation: http://arduino-distance-sensor-library.googlecode.com/hg/Documentation/latex/refman.pdf
Inheritance diagram: