1. Install the Sony Ericsson GPRS modem scripts, then
2. Set up the Bluetooth PPP to use one of the Sony Ericsson GPRS scripts -- you may need to experiment to find the right one
3. Enter the PPP configuration using the settings for your carrier from the list.
The list of settings, and the scripts, are available on
http://www.taniwha.org.uk/gprs.html. I used them to try my T637 on AT&T and it worked fine.
tooki