Hi all,
Mattreichenbach is probably on the right track here with a reset of settings. I think I've determined the issue has to do with cached account information and inconsistencies for the account name. This seems to crop up when I've changed my password and it hasn't propagated fully to all the servers, devices, certificates, etc.
Hopefully many here are on their way to restoration of service by now but it's clear that a lot of people are having the same issues. Very frustrating and definitely something Apple needs to resolve: incoming mail server field grayed out, incorrect autopopulation of different fields, other unanticipated behaviors. If you're still having issues, though, here's what worked for me...
First, head to the iCloud preferences pane in System Configuration and choose "Sign Out". When I did this it prompted me with a number of "are you sure" type questions about retaining information on my local machine. I chose to delete/remove the info each time simply because it should all be restored by the cloud and I didn't want to risk a massive duplication of my data... I will say, I'm back up and running with no issues so I suspect you can make the same delete/remove choices... But use your own judgment. I don't want you to lose any data (ie.. please use care as you do this because I cannot bear the thought of causing anyone to experience the pain of data loss).
Once you've signed out of iCloud, restart your system. When you get back in, head to the Mail, Contacts and Calendars section of your System Preferences and add your account back by choosing the brushed aluminum "iCloud" button. When it asks for your account name, use your @iCloud.com email address. I am all but certain my issue had started because one of my devices (iPhone or whatever) had been set up with an email alias specified instead of my @iCloud.com address.
Apple, if you're reading through any of these issues (there are TONS of users having this same problem). Take note, that your icloud service somewhat frequently is not responding and yet tells the user that the password is wrong and this prompts people to be changing their passwords unnecessarily. This has happened to me on numerous occasions. Also, I noticed that last week's outage corresponded to a VERY similar outage exactly one year ago to the day. Sounds like planned maintenance to me and I think you could do a better job notifying folks so we're not wasting HUGE amounts of time troubleshooting a problem that we have no hope of fixing.
Hope that helps some of you!
Terry Mullane
Washington, DC