I need some help. I have this iPhone 3G downgrade before to use it with Tmobile.. Worked fine and all, but now I just want to put it back to the original at&t and not to deal with cidya, I want to do my regular updates via iTunes. Any one knows how to do this? i tried several things and end up with the same error code 1015.
Thanks for your advice.

Ignore the 1015 error,restore your phone and then try again.

To unjailbreak the phone, just restore a current (non jailbroken) .ipsw of a version of iOS that Apple is still signing. Since you're on the 3G, I think the most recent version that you can use is 3.1.2 or 3.1.3 -- but it's been a while. I'm certain that 4.0+ was not legitimately released for the 3G, so start looking at 3.1.2 or 3.1.3.

Error 1015 can happen if you're downgrading to a version of iOS that is older than the baseband you currently have installed on the phone. In most cases, it's just a little hassle and nothing more -- once the restore has completed, you can use TinyUmbrella to 'kick the phone out of recovery mode', and the phone will reboot and be unjailbroken and locked.

Depending on what baseband you have, however, there may be more steps. Can you let us know what version baseband is on the phone?