After a considerable effort I’ve managed to get GPRS working on my D600 on the Cingular network. This has been a real pain but finally it works and I can get to Gmail from anywhere.
Here’s the settings you need:

WAP BROWSER SETTINGS:
————————————
(account settings details for connection)
go -> menu/browser/profile settings/media net

Profile Name : “Media Net”
Home URL : “http://yahoo.com” (any wap site NOT “http://device.home”)
Bearer : “GPRS Only”
Proxy : “Disable”
GPRS Settings
DNS1 : “66.209.11.61”
DNS2 : “0.0.0.0”
APN : “wap.cingular” (all lowercase)
Login ID : “WAP.CINGULARGPRS.COM” (all uppercase)
Password : “CINGULAR1” (all uppercase)

(which account to use to connect – should be the “Profile Name” of the one you created above)
go -> menu/browser/current profile/
“Media Net” (selected)

MMS SETTINGS:
————————————
(account settings details for connection)
go -> menu/messages/settings/multimedia message/mms profiles/profile settings/Cingular mms

Profile Name : “Cingular mms”
Server URL : “http://mmsc.cingular.com”
Proxy : “Enable”
version : “1.2 (295kb)”
GPRS Settings
IP ADDDRESS : “66.209.11.32”
PORT : “8080”
APN : “wap.cingular” (all lowercase)
Login ID : “WAP.CINGULARGPRS.COM” (all uppercase)
Password : “CINGULAR1” (all uppercase)