I just discovered that I can't get a replacement phone headset from PalmOne, but I can use one that Fellowes makes for the Nokia 8000 series cell phone. They also make a converter for the Nokia that may (or may not--I haven't tried it yet!) allow you to use the more popular kinds of cell phone headsets.
Perhaps that converter thingy could be used to make the TW work with other headsets and MP3 software somehow?