Connect to iTunes to fix it!
Situation:
My dad had this issue with his iPad 2 with AT&T 3G. It has never been activated to work on the AT&T network. When he picked up his iPad yesterday, it prompted him with the same "Activation Required" message that prohibited him from using his iPad until it was satisfied. It would error out stating that the activation server was temporarily unavailable.
Steps taken to fix it:
I removed the SIM card. That didn't fix it. I reinserted the SIM card. That didn't fix it either. I connected the iPad to my computer and opened iTunes. (The iPad may ask if you trust the computer.) The activation went through immediately, and everything was back to normal on my dad's iPad.