I got T-Mobile to enable GPRS ("T-Mobile Internet") on my phone the other day. It works alright with the t68i and the PowerBook. I used Ross Barkman's "modem scripts". With the right script with the magic "use GPRS" commands I just had to choose the correct "CID" and enter "internet2.voicestream.com" as the phone number. I also setup a "Data account" on the phone; I'm not sure I actually needed to do that.
A few odd things:
I can't figure out if I could use the $10 "T-Zones" service rather than their "Internet" service which is more. I only need web access and ssh. Can they really tell if you use the phone as a modem rather than just use data from the phone? That was what the customer service person told me.