We stopped and started PlantronicsURE.exe on this system (and a second one with a CS50 headset that also had OutOfMemoryExceptions in it's log), and both systems are now working.
The Windows 7 system that produced the dump and logs for this thread was rebooted a number of times yesterday afternoon (4:30PM Central or so), so it doesn't seem like this is an accumulation of a slow thread leak over time.