I had a lot of trouble figuring out how to use my brand-new Sony Ericsson T68i as a wireless modem with my AT&T GPRS Wireless Service. I had hoped that I could just "dial in" and use my thousands of minutes to get dialup service like any other modem connection. Unfortunately, AT&T wireless doesn't support this and it will disconnect almost immediately if you try. You must use their GPRS service and very few people know how to set it up correctly because it's so cutting edge.
Finally, I got it to work, but had to go to numerous sources, including several websites discussing related topics, my local Apple Store (which couldn't help either), and telephone calls with AT&T Wireless Services' Mobile Internet division (866-293-4634). Note that I was told they don't support Macs or Bluetooth. As a result of my difficulties, I wrote up a set of clear (I hope) instructions.
This is definitely one of the coolest things to do with a bluetooth phone and a real "showoff activity" for your PC friends.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030310183119445