Wow, that was easy. I had to tweak my BIOS to enable the IRDA port. Then I spent ten minutes hunting around menus looking for how to enable the IRDA but it turns out it was on, by default. You can't turn it off (as far as I can tell).

So, all I had to do was enable IRDA on the phone, move it in range of the computer, and foomp!, a "generic cel phone modem" appeared as COM4. No need to install any drivers or anything. I created a new dialup account for *99#, called it with no username or password, and there I was, surfing the net. Without paying the extra charges. There's no way T-Mobile will let this last forever, but it kicks butt for now.

Edit: in case anybody else tries this and it doesn't work as smoothly, Google helped me find more detailed instructions in a nice PDF from Nokia. The only missing instructions there are the magic *99# phone number.