I have successfully gotten a USB tether between my macbook and my bb 8830.. since i kinda figured it out on my own, and i remember finding no help on the interwebs, i figured i'd post it here since this is a top result from google on the subject.
i am on verizon and this uses verizon software, so im not sure if it will help other users... although it uses the os x network preferences, so i would try changing those settings to see if it will work with other access.
first, I downloaded VZAccess Manager and even though it claims not to support my bb on the mac, i got it to work.
i have a backup of the installer at http://www.projectbutter.com/bb/vzam
please don't download it if you are not an authorized user of the software. that being said, when i looked in the contents of the installer package, inside contents/ i found archive.pax.gz, which installer uses as a model for the files to copy.
inside the folder structure, i found the files that get copied to /system/library/extensions and, lo and behold, the was a kernel extension called blackberryUSB.kext and (and blackberryUSBdev.kext) im not sure if a normal install would have placed those files where they belong or not, because by that time i had already manually moved them into the indicated folder. i was hoping this would allow me to use mac os x's built in menu extra as i do with bluetooth, but it still requires the vz access manager to connect (i noticed a "initializing device" status message, and feel that's the missing link.) a small prive to pay for USB speed broadband access for my mac via my blackberry.
I hope this comment finds other bb/mac users who have all but given up their quest to get online at full speed. please post your results with changing the dial-up number, user and pass to conform to other access providers if you get it to work, i have no idea how that will go, but it looks slightly promising. plus.. if anyone figures out how to get internet connect / the modem menu extra to skip the need for vzaccess manager, please kindly let me know how at butter(a)mac*com also, if this post helps you i'd love to know it was worth writing it up, so please drom me a mail and let me know.
best of luck, and happy roadwarrioring,