I'm in the process of moving, and where I am, I have NO internet access except for my BB, I've been using the PDAnet app (PdaNet -- Use your BlackBerry as a USB Modem ) and I've had better throughput with it than with Tether.
NOTE: I have mentioned this before, but AT&T seems to be charging if you use the "Use your BB as a Modem" setup many people suggest. At least, they did for me. I assume it's because your BB is acting as a modem and your PC is talking directly to the WAP, where as, if you go through one of these programs, the BB is acting as a proxy server and making the requests to the WAP.