I finally got my Treo 650 to work as a Bluetooth modem for my Mac

To get your Bluetooth-enabled Mac to use your Treo 650 as a GPRS Modem, you'll need this article, and this page of GPRS settings for various operators.

So, how well does it work? Quite well, actually. Speeds on the USA T-Mobile GPRS network aren't great, but it's better than dial-up. Email and "clean" sites (e.g. Google) feel quite responsive. Inefficiently coded sites (e.g. Amazon.com) do feel sluggish, but I imagine that's true for dial-up users as well.