The best (and probably only way) to do this is to get a copy of Tether for BB @ Tether BlackBerry | BlackBerry Modem | Tether. It retails for $50 but you can get it for 1/2 price when it goes on sale. Pay once and never pay for tethering again to your carrier.
There are some workarounds out there but the carriers know about most of them. If you get caught, be prepared to pay a large bill. Tether for BB is the best way to do this and the carriers apparently have no defense against the use of this program.
.... the carriers apparently have no defense against the use of this program.
Not quite correct.Aside from all the technical stuff they can do, they also have a very simple method of detecting tethering - by the amount of data used.
All carriers can tell you what the average BB user uses for data. Use substantially more and you can bet that the carrier is going to give your account a closer watch.
The Tetherberry works great on my phone. I get comparable speeds to my WiFi connection from my computer. It has already paid for itself in the couple of weeks since I have had the application.
There is another option thats not quite that expensive, i use pda net from junefabrics.com. After a lengthy free trial period it was only $28 and i get unlimited tethering, i only use it when i cannot find a wifi hotspot, which in this city is rare, but its there if i need it.