iOS 7 Activation Error (legal)
I had the same issue this morning, it's because the beta has expired. You shouldn't need to restore, you can just update to the latest firmware (this will leave all your photos/messages/apps etc as they are):


Download the latest .ipsw for your device from http://developer.apple.com/.
Update to the latest version of iTunes.
Plug your phone into your computer and select it from the devices on the left in iTunes.
Alt-click on "Check for Update" button and choose the downloaded .ipsw.