I just got it working! I'm using 4.5.0.52
It may also work on 4.2
So, download the modem script from:
Wireless Internet: BlackBerry Curve as Bluetooth Modem for OS-X Mac • Perishable Press
The instructions are for OS 10.4 I think, so it doesn't quite follow with 10.5.
Basically, I got it to work after removing any old blackberry devices from the bluetooth menu and the network menu. Then pair the BB & Mac. Create a data connection and for modem, select "Other" and Blackberry 8300. Phone number choose wap.voicestream.com (or whatever your APN is - the modem script doesn't actually dial this, it dials the usual *99***1#, but uses this field to get the APN information). Username/password are guest/guest (might be different for providers other than T-mobile).
Go to advanced in networking preferences for bluetooth and enable error correction, send PPP echo packets, disable TCP compression.
Should work!