So I got my phone 1 day early. I plugged it into iTunes, got it all synced up and setup. Then it gave me a short little message saying something along the lines of “Activation will take a little bit of time.” Thinking nothing of it, I set my phone aside and went back to work. An hour later, my iPhone 4 still wasn’t activated on the cellular network, and my iPhone 3GS was still working. A quick google search revealed that many users were having activation problems.
How to Activate Your iPhone 4
If you’re having problems with activating your iPhone 4, here’s the way to get the new one to activate. Turn it off, then back on. That should do the trick. If that doesn’t work, try the following: Go to http://att.com/Activations