According the Plantronics website (and I quote directly).
The microphone plug on our .Audio Analog headsets will not work on Macs. It will physically fit into the Mic input jack on a Macintosh, However it will not function. Basically the digital soundcard does not recognize the analog signal coming in from the headset. A way to convert that signal is the Griffin adapter or the DSP50.
I won't be be purchasing an adapter and will in most likelihood return the item, but on the off chance is their a piece of software out their that could make the two work together?