Sprint has had this for some time now on both their 2g and 3g networks.

Their "PAM" (phone as modem) plans start at 39.95 over whatever your existing plan costs. Note that their $99 "everything" plan does _not_ cover PAM.

Almost all of their phones that are data capable can be used PAM. You will need a USB cable to tether to your computer. They will tell you that you need some software, but all you need is the USB driver for your phone and to set up dial-up networking to call *777 and you're in.

Some of the older Sprint phones (my old Sanyo 4920 was one of them) could act as a PAM without any additional plan, but it was on a slower network. It was still good enough for checking e-mail for a few minutes when no other Internet service was available.

The downsides: As you noted, your phone is not available while being used as a modem. Most of the USB cables will not charge the phone so you will be limited to the talk-time battery life of your phone. Finally, I noticed a lot of latency (turn around send/receive time) with the PAM. Steady-stream downloads were pretty good though.