Thanks Don,
It seems that Apple made a change to the USB audio interface driver in one of the updates to OS X 10.4. As a result, detection of sidetone support for this device (and some others) did not work for any version of 10.5 through 10.8.
However, we figured out how to enable sidetone manually by issuing USB commands directly.
We are very pleased with this device, particularly the component quality and the support & documentation from TI.