Your iPhone could not be activated because the activation server is temporarily unavailable
The two major causes of this are:


Network related - change the network used to connect
iOS related - something is amiss with the code signing / provisioning


The most likely cause of software is a beta. Apple disables the iOS betas soon after the release happens. If you never manually installed the official release, this can happen. If not, consider backing up your device after checking a couple networks and reinstalling.

If you need to get a beta off, do the following to fix it:


Hold the Power and Home buttons until the phone reboots and you see the Apple logo.
Release the Power button but keep holding the Home button.
Release the Home button when you see the iTunes logo and the connector below it.


Do this while connected to iTunes. Click Restore. Hopefully you have a recent back-up.

Note: It is a large update and will take a while. Apple servers are currently getting slammed.