I was having difficulty obtaining a cellular signal on my Ipad (not the first Ipad but not IPAD 2) after I updated from IOS 5 to 6. I backed up my ipad on Itunes and restored the settings.
I get the white screen "Hello" prompt, choose English, choose USA and set my WIFI.
From here it continues to state that the activation server is unable to connect.
I attempted not to use WIFI and work straight from my ITUNES using the USB to my MAC and it continues to state the same thing.
This iPhone troubleshooting guide may also be able to resolve your iPad problem:
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