I use something called a Chatterbox HJC50 to communicate rider to passenger on a motorcycle. The chatterbox has a cable for cell phone attachment. It works fairly well with a Treo650. I will say that it sounds better than any normal headset I have tried with my Treo. The drawback is that there is no button to answer calls and you still have to use the buttons on the phone in some way. But it works with excelent quality and the nice side feature is that two people on the Chatterbox can both participate in the phone call.

Chatterbox link: