Hi Tim,
Great work! I will be trying this out Real Soon!!!
There are quite a few "HC-S04" labelled modules around. Have you tested any variety??
Here are a couple I have: http://goo.gl/VZkSy
The one labelled SRF-06 is said by the supplier to have temperature compensation. It does have a different circuit board / chips.
I'll send you one of each of the HC-SR04 and SRF-06 modules if you want to test them and see if you see any differences.. drop me a PM or email terry@yourduino.com
I'm writing a library for these: http://goo.gl/FWMjI and I'd love to try some cool mapping ideas with your code...
DISCLAIMER: Mentioned stuff from my own shop...
OTHER QUESTION: Your example is for Arduino IDE 1.0 Does the library work for both 0023 and 1.0??