Apple has confirmed that select iCloud services are down for users, including Documents in the Cloud, iCloud Mail and Contacts, and Photos. Services like iChat, Maps, iMessage, FaceTime, iTunes, App Store, Apple ID, and Find My iPhone are still working fine.
Apple did not give any reason for the outage, but you can check on the status of iCloud through Apple's system status page.
Is iCloud down for you?
Update: Looks like iCloud is back up and running!