What worked for me was something similar. I had the IOS6 beta version on my iPAd3 Wifi, running iTunes 10.6.3 and it suddenly stopped working and told me to activate iPad?? Each time I tried to activate the iPad either connected to my Mac via iTunes or over Wifi it said that the activation server was unavailable. And there was no issue with my Wifi conenction whatsoever. Here's what fixed it for me:
1) Shut down iPad completely
2) Delete any iPad IPSW files on your mac/PC -- You might be able to skip this part
3) Upgrade version of iTunes software from 10.6.3 to 10.7 --- For some reason iTunes thought I was running the most current version of software even though I was running 10.6.3 and 10.7 was avaialble. I had to go to apple.com/itunes and manually download the latest version rather than updating via iTunes app.
4) Restart Mac
5) Open iTunes
6) On iPad hold down the Home button and while holding it downt plug into the iPad the cable thats connected to your PC - You should see an iTunes and usb cable images on your iPad
7) Follow prompts in iTunes to restore your iPad to IOS clean version (IOS6)
8) Restore your iPad from a previous backup - I chose to setup a new iPad just to be safe
Hope this helps....