If you have AT&T (with say an iPhone) and get the Android Dev Phone, you can use your iPhone’s Sim with the G1 as it is both hardware and software unlocked.
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:
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.