updated 02:51 pm EDT, Wed June 20, 2012
Affected users may be totally cut off
(Updated with resolution) iCloud services are currently being seriously disrupted for some users. On Apple's official status page, the company simply states that as of 10:45 Pacific time, "Users may be unable to access iCloud services." One MacNN reader explains that he has been unable to access the likes of email, Calendar, or even iMessage during the downtime.
Anecdotes from other users support the situation, but issues are not universal. Some people are experiencing no problems whatsoever, possibly suggesting trouble with specific servers or connections on Apple's end. iCloud glitches rarely affect all services at once; the current incident is, however, the third one this week, following problems with Photo Stream and later Mail and Notes.
Update: The iCloud status page now indicates that the outage is over as of 13:06PM Pacific time.