I had the same problem after trying to use fMMS. Every time I tried to manually
connect to AT&T connection it'd take 2s or so to prompt the Connection Failed
message. Rebooting didn't help. After comparing two backups (the
configuration backup is stored in .xml files in a zipped file. kudos to the maemo
team for using open formats) I noticed that the AT&T connection settings like
username, password, etc were all erased. And such configurations were
automatic, so I never had to enter them.
Go on Settings, Internet Connections, Connections, select AT&T Internet and
select 'edit'. Then select Next and you'll see the access point name, user name,
etc. The settings:


Access point name: wap.cingular
User name: WAP@CINGULARGPRS.COM
Password: CINGULAR1


preserve the caps on the user name and password.
let the password prompt off. Select Next and then advanced. In advanced
proxy usage should be off and auto-retrieve IP address and auto-retrieve DNS
on.