There's a Mac BlueTooth phone program that works with most bluetooth phones and works pretty well with the Treo 650.
http://www.reelintelligence.com/BluePhoneElite/
It's great. A caller ID pops up on the screen when someone calls. You can set the proximity, monitor your signal strength and a bunch of other features.

Does anyone know of equivalent Windows software? Hopefully something that can pair with Palm Desktop?