This was the response I got back from Tech Support at Griffin:
"Actually, not all functions of that cable are compatible with mobile devices other than iPhones. The handsfree talk functionality uses the iPhone's 3-ring input/output port. So with the HTC Thunderbolt, it will work fine as an auxiliary audio cable, but handsfree talk will not be available.
Please let me know if you have any questions or if I can assist you in any way. Thank you, and have a great day!
Dylan Ross
Griffin Technology
Technical Support"
Does the Thunderbolt really not have a 3 ring input/output? I thought for certain you could use an in-line microphone with it. Anyone have any knowledge they can drop?