I just created this guide to tether your internet connection from your Blackberry to your Mac via Bluetooth.
Tested with OSX 10.6.2 and Blackberry 8520.
I noticed a lot of people wanted to know how to do this but were having problems.
I have the RIM modem script if anyone needs that too.
It happened to be on my mac already so I'm not sure if it was there by default.
It might be installed when you install the Mac Desktop Manager.
There might be a few variations in the positioning of menus and input forms, so you might not see the same layouts as on my screenshots but the information is still relevant.
As long as you put the information in the right fields, it should work.
If you come across any problems, let me know and i'll try and sort it out for you.
Do you have a USB cable for you BB?
We'll try tethering via USB to see if that works, incase it's a bluetooth problem.
If you connect it to you mac via USB and then goto System Preferences > Network.
The mac should recognise the BB and make a new Service called "RIM Composite Device".
Input the information like you did for the bluetooth set up and test it again.