I Recently was playing with my Android.

After installing a new ROM I noticed my data network was not working.

After some searching around I found the following:

 

Basic Settings

AT&T
APN: wap.cingular
Username: WAP@CINGULARGPRS.COM
Password: CINGULAR1 (must be in CAPS)

 

Expanded Settings

Name: AT&T
APN: wap.cingular
Proxy: leave blank
Port: leave blank
Username:WAP@CINGULARGPRS.COM
Password:CINGULAR1 (yes this is in CAPS)
Server: leave blank
MMSC: http://mmsc.cingular.com
MMS Proxy: wireless.cingular.com
MMS Port: 80
MCC:310
MNC:410
APN Type: leave blank