A lot of people have complained about the loss of their AT&T 3G data connections on their iPhones after an iOS 4 update. TiPb has posted a fix that might work.

• Use an iTunes with Administrator rights, or create a new admin ID.(This stops iTunes from looking at primary user info inside iTunes as that seems to be the issue)
• Open iTunes 9.2 and plugin your iPhone, click on Restore, it will download the iOS again and load it on your phone.
• Setup as new phone and after it sync’s, check Safari for internet access.
• If that works, then login to your normal ID and rename your iPhone and RESYNC all your data.

If that doesn’t work out for you, you can try the good old default resetting network settings by going to Settings > General > scroll down to reset.

Do comment if this works out for you.

[via TiPb]