When you insert your Sim and then start up the Dev Phone for the first time you will come to the first page of the setup.

Click on the Menu button and click Add APN.

Now enter in the following information:

Name: att
APN: wap.cingular
Password: CINGULAR1

Leave all the other fields blank.

Now click on Menu again and choose Save.

That should be it! You can now use your Android with your AT&T account.

If you don’t know the Dev phone is a G1 that is fully unlocked (hardware, and software) and allows itself to be loaded with unsigned firmwares.

Here are some pictures as well as a few comparing it to the all mighty iPhone.