0n 10.2.5, use the Bluetooth Serial utility (installed in Utilities folder) to create a new port. Give it a name like "BTAccessPoint" (name is one word). Hit "Outgoing," and pick the desired Bluetooth access point by clicking on the "Select Device" button. Discovering your access point services, select "LAN access using PPP." Note: to be able to discover the access point, make sure the access point is properly set up, and that, in particular, it does not filter out your MAC address...
Select the "Show in Network Preference" option, and eventually other encryption or authentification settings your Access Point may require (not tested with that option on on my setup).
Select RS232 on Port type. Close the setup window. Now the newly created port should show up in the port list (with BT AP name) and be selected. Quit BT serial port utility.
Go into Network preferences. On opening, it should show a newly detected port (BTAccessPoint). The TCP-IP tab should be set to "Using PPP." On the PPP tab, leave the phone number blank. Eventually fill in username and password info if your Access Point PPP settings require them. On the Modem tab, select "Null Modem 115200" modem profile.
You are done! Make sure this nework port is *before* other eventual network interfaces, since this order is the load order. Select the port in Internet access app (or menu), hit connect, and there you go! Good luck.
[robg adds: I haven't tested this one, as I have no Bluetooth enabled phones.]

