I can confirm what ashzx said about Sprint. I was able to tether my N810 to my Palm Treo 755p without any additional software, drivers, etc., for at least 5 months then all of sudden it failed all the time. I was able to confirm that Sprint did change something, which I find a little annoying since I contacted my Sprint rep and asked if I could now get a PAM (phone as modem) plan, and he said that it was still not available (I'd asked numerous times before). To tether my Windows notebook computer to my Treo, I had to use an app like USBModem or PDANet. But I never had to do that with my N810. Now I have to run USBModem on my Palm in order for my N810 to successfully tether. I imagine that you're encountering the same thing.

I don't know much about Blackberry or what hacks are available for the various models, but if you just can't get it working you can always try Mobile Stream's Blackberry Tethering Solution app for $9.95:

http://www.mobile-stream.com/modem_bb.html

It's a better deal than USBModem (by the same company) -- $24.95. That about killed me to have to pay that for a Palm app, but not having the ability to tether was rendering my N810 all but useless.

-- Chris