[[NSNotificationCenter defaultCenter]
    addObserver: self
       selector: @selector (iCloudAccountAvailabilityChanged:)
           name: NSUbiquityIdentityDidChangeNotification
         object: nil];

If a user signs out of iCloud,such as by turning off Documents & Data
in Settings,the ubiquityIdentityToken method returns nil. To enable
your app to detect when a user signs out and signs back in,register
for changes in iCloud account availability. – Apple Documentation

http://developer.apple.com/library/mac/#documentation/General/Conceptual/iCloudDesignGuide/Chapters/iCloudFundametals.html