Since a lot of us came from the Treo 300, this may be old news, but I just figured it out, so I'm excited!

Being a cheap *******, I plugged the Treo 300's factory handsfree headset into my Treo 600 and, no tricks required, you can listed to audio with it. The sound quality of an MP3 is quite surprising.

When a call comes in: Answer the call. The audio from the other person comes in through the earbud, but the microphone on it does not work. However, the microphone on the Treo600 itself is picking up audio, so just hold it close enough, upside down preferably, near you and you have a free, small, perfectly useable audio solution.