I've only tried this with T-Mobile, but it should work for anyone. It should also work with other Nokia Bluetooth phones, but that hasn't been tested either.
Pair up your Bluetooth devices the usual way. Set up a new Bluetooth modem port in your Network preference pane. For the modem type, set "Nokia Infrared." Disable "wait for dial tone" and error correction/compression. On the PPP tab in PPP Options, turn off PPP echo packets and turn on TCP header compression. Click "OK" through everything, and enter your PPP info as normal. If you're using T-Mobile internet, set your dial up number as *99# and leave username and password both blank.
That's it, you're all set!
For what it's worth, the same setup will work for an IRDA port to use the phone as an IRDA modem. Only trick is that on the 3650 you need to set up the "connection" of IRDA first (turn on IRDA) and then attempt to connect from the Mac after activating the "modem" program on the phone. The first attempt will fail, but then it'll work fine. T-Mobile documents the whole thing in their support pages except for the Modem program, which is essential to make it work.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030908135213353