Per my previous post I ran in to a blocking error with the USB cable method. I picked up a cheap USB bluetooth adapter and I now have a T-Mobile provided DUN tether working with my 8700 in Kubuntu 8.04 Linux (I'm posting this from it now)!
Here are a few things that are particular to tethering a Blackberry 8700c (unlocked from AT&T) using TMobile's service which differ from SDNick484's instructions:
First off, the steps at Blackberry Curve 8310 as Bluetooth Modem on Ubuntu Hardy « pegelinux
were fantastic. I'm going to use their naming convention of gprs and gprs-connect-chat instead of bberry and bberry-chat, respectively.
For my configuration, rfcomm.conf needed to be set to use channel 0. I determined this by running:
This told me that my phone was using channel 0 so my rfcomm bind command looked like this:
sudo rfcomm bind 0 00:11:22:33:44:55 1
Note that this is a slightly different syntax than other online instructions, but it seems to work OK.
In the gprs (bbery) ppp config file, T-Mobile does not require a username (not even a phone number) so erase the user line entirely. I also set the IP address to :169.254.1.1 which made a lot of console errors go away later.
In gprs-connect-chat (bberry-chat), the following two lines are unique to T-Mobile:
OK 'AT+CGDCONT=1,"IP","wap.voicestream.com","",0,0' \
In my case, I had to change my phone's APN (Options -> Advanced -> TCP -> APN field) to wap.voicestream.com as well (with user and password blank because T-Moble just plain doesn't use them). This was probably only required for me because I wiped my phone when moving it from AT&T to T-Mobile. You can double-check your service book to see what your APN is set to.
Once I got that all set up right I was able to run pppd call gprs and everything was happy!
Again, it's probably easiest to use the T-Mobile specific settings with the guide I attempted to link to above as it is very complete. Thanks to everyone who posted information on how to get things working, in particular tcolar and SDNick484. If I happen to get it working through USB I'll be sure to post how I did it but for now I think Bluetooth is a perfectly fine solution.
Best of luck to those who follow,