Back in March of 2006, I wrote a blog post on my old blogger account on using DUN via bluetooth to get online with a Mac. Then, I wrote the following:

1) Turn bluetooth ON on your phone but leave DUN OFF. This is critical. It took me a dogs age to figure this out. It isn't documented anywhere.
2) Go into your bluetooth menu on the laptop and setup bluetooth device.
3) Choose phone.
4) follow the instructions on the laptop. It will give you a passcode which you will enter into the Treo
5) Turn ON DUN on the Treo
6) Open Internet connect on your laptop