I sent this info to Apple Feedback last week, at the request of a Tech Support whiz, but it seems that we found a glitch in the iCloud rollout that's causing a number of bedeviling (and until we figured it out, supposedly not yet documented).
This resolution only applies to those folks who "once upon a time" had a Tools or Apple ID or e-mail address for whatever reason that used the old "mac.com" construct.
Those people, somewhere along the way (MobileMe rollout) were given a "me.com" address -- but the old mac.com addresses were never vaporized, as they should have been.
Soooooo, along comes iCloud. If you've got old mac.com addresses associated with SOME of your stuff, me.com associated with SOME of your stuff you've got issues. Now, depending on what you used for setting up iCloud will determine just how internally confused your Apple environment is.
A completely avoidable issue back when they converted all those mac.com to me.com; a completely avoidable issue in the implementation instructions of iCloud (and another chance to vaporize the interchangeable .mac/me ambiguity).
While you can use .mac or .me for iTunes, you CAN'T mix and match them within iCloud. Go make sure that your calendar settings and iCloud settings are using EXACTLY THE SAME Apple ID. It may solve your problem.
This fix also untangles some of the messaging failures.
Completely avoidable, Apple missed both chances. Now they're paying for it with our time and displeasure, not to mention their support costs while chasing this little ghost. No official fix yet that I'm aware of.