I'll have to find out which softphones this headset works with when the exhibit hall opens in 20 minutes. Hopefully, the list includes Xten, Skype, and other major VoIP softphone clients.
I met with Plantronics, and they told me they will indeed work with Skype, Xten, Avaya, and several other softphone clients. They are working with the vendors to provide tight integration, including ringing the wireless headset. I asked them, "What about outbound dialing? There is no keypad on the headset, so do you plan on implementing speech-rec dialing?" They told me third party vendors are indeed working on it. So expect to be able to just say, "call Mom" and it will initiate a Skype call, SIP call, etc.
You can buy the Plantronics CS50-USB headset on Amazon pretty cheap - around $180. Also check out the good Amazon customer reviews on this headset. I hope to review this headset myself soon.