iCloud (and others) greyed out in settings
I upgraded from my iPhone 3GS to an iPhone 5 (IOS6) and found that iCloud, Twitter, Facebook and Facetime settings were all greyed out.
I did the usual cycle of the power to no effect.
I recall that when I upgraded to IOS 5 that I hade declined to accept a EULA for iCloud (not knowing what it was at the time) until I had researched what it was all about. - Then promptly forgot all about it.
So, I RESET ALL SETTINGS in the hope of clearing out the flag that was blocking just these settings and - voila - all fixed.
No loss of texts, emails, photos or any apps or other data. All I had to do was fix a few settings like ring tone, wallpaper, WiFi login etc and away I went.
Hope that this works for you.
C