OK, I don't have a 650 to test with yet, but try this. I created this script with Modem Script Generator. Drop it in the modem scripts folder located at "/Library/Modem Scripts" and select it as your Bluetooth modem script in network prefs.
Then, use these settings in Internet Connect:
Telephone: **99***1#
User:
WAP@CINGULARGPRS.COM
Password: CINGULAR1
Also heed this note from Ross Barkman's page:
"Cingular's WAP APN does not support compression, and only allows 2 MB to be transferred before disconnecting you."
http://www.taniwha.org.uk/gprs.html