Apple’s System Status page is showing multiple iCloud services with ‘issues’:
Services listed as being affected are Photo Stream, Documents in the Cloud and Backup. The services have been having problems for nearly five hours already, and Apple has not yet provided a timeframe for a fix.
(Thanks, Guillermo.)
Update 8:28 AM: The issues are persisting some seven hours after Apple first acknowledged them, but the company has now updated its status page to note that less than 1% of users are currently being affected.