I had the same problem and after researching it, our friends at Cingular have created a new "BlackBerry Tether" plan that you must activate for it to work. When I called in, the Rep didn't know about it, and when I told her it was the $79.95 plan, she sorted by price and saw it. So this plan replaced my unlimited BlackBerry data plan, and once she put the changes in, it just worked.