If you got the "scary" message that you could not activate iPhone 4S neither by Wi-Fi or iTunes, I want to share my experience:
I decided to reset my iPhone 4S, and used "Erase All Content and Settings" under the reset menu (Settings/General/Reset), I was hoping that the erasing just does that, but for me the nightmare started!
I was taken to the activation screens, and begun with the welcome one, choose language, choose location services (later on it also disappears) and selecting Wi-Fi and/or iTunes for activation.
I chose a Wi-Fi network the process started, and gave me a message that it will take about three minutes to activate iPhone, few minutes later a screen with this message appears: "Your iPhone could not be activated because the activation server is temporarily unavailable."
Then, I decided to try the activation using iTunes…
Using iTunes, after few minutes, it shows the following message: "We could not complete your iTunes Store request. The iTunes Store is temporarily unavailable. Please try again later."
I tried several times in both ways, trying 4 different Wi-Fi networks and 3 different computers in the whole process, and I was going desperate! I started to find out some info about this trouble over the Internet, and guess what? I was not alone! But I did not find any answer or solution either!
Finally, I joined Apple Support Communities, and started searching with no good luck at all, until I found (many hours later) this old (2009) solution:
+Do a DFU recovery:+
+1. Open iTunes.+
+2. Connect the iPhone+
+3 Press both the home and sleep/wake buttons until the Apple icon appears (ignore the red slider if it shows).+
+4. Continue pressing the home button alone until iTunes sees your phone in recovery mode.+
+5. Follow the prompts to restore the iPhone.+
(THANKS A LOT!!! modular747 and Easily Easy, you saved me!!!)
Then I suggest everyone to use this, of course it will erase everything, but you may be able to recover your data from your last backup, or you can also choose to configure iPhone as a new cell phone.
The process took me about 92 minutes, but it was worth it, now I own an unlocked iPhone 4S (it was an AT&T when I purchased from Apple Store in November) and is working perfectly!
Hope it is useful for all of you facing this trouble!
Good luck and enjoy your iPhone then!