I spoke with Apple Tech Support and they are looking into the issue.
I took a look at the logs (not sure why I didn't do this before) and it appears that the CalendarAgent is continually crashing. I booted from a test install of ML on an external drive, no issues with iCloud & Calendar.app sync. Moving the CalendarAgent executable over and repairing permissions did not help, issue persists. Is there a file somwhere that this needs to be fixed? Seems like the host information has been set to nil.
Process: CalendarAgent 
Path: /System/Library/PrivateFrameworks/CalendarAgent.framework/Executables/CalendarA gent
Version: 1.0 (47)
Code Type: X86-64 (Native)
Parent Process: launchd 
User ID: 501
Date/Time: 2012-08-03 13:51:04.592 -0400
OS Version: Mac OS X 10.8 (12A269)
Report Version: 10
Crashed Thread: 10
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Application Specific Information:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'The value for 'host' should be non-nil and non-empty'
terminate called throwing an exception
For what it is worth I've got the same issue right now. It is happening on both of my machines running 10.8. I just upgraded to 10.8.1 in hopes for some relief but no dice. I am experiencing this issue in Contact, Calendar, and Reminders. I am getting the CalendarAgent crash about every 45 seconds.
I spent almost 2 hours on the phone with Apple the other day. I ended up sending them a lot of crash data and such and their engineers were supposed to look at it. I haven't heard back yet but I'm real close to abandoning iCloud over this.
I have sent Apple crash data as well. The support agent I've been dealing with called me last week saying that the engineers have determined that the issue is with my iCloud account.
I have tried a test account and I can import all my calendars with no issues, sync works fine. Hopefully this will be resolved soon.
Just out of curiosity did you have access to iCloud through the Apple Developer center prior to the official release?
I had this problem and just spent two hours trying to fix it. Here's what I have learned and how I solved the issue.
These things solved a lot of people's issues:
- Restart your computer and iPhone.
- Turn off iCloud on your iPhone and turn it back on.
- Turn off iClound on your computer and turn it back on.
However, they didn't work for me at all. My problem was with CalendarAgent, the program that syncs reminders and calendars. You can tell if you're having my problem by doing the following steps:
- Start the Applications > Utilities > Console application.
- Click on the Filter box in the upper right and type in CalendarAgent.
- If you do not see a bunch of entries withe the words "crash" or "Terminating app", then I can't help you
- If CalendarAgent is crashing and you see erros containing the phrase '
The value for 'host' should be non-nil and non-empty', then you have my problem! Try logging into iCloud.com, creating a new calendar, and deleting your other calendars. You can leave subscriptions alone. This removed a corrupted calendar from my iCloud account that my Mac failed to sync over and over which was crashing CalendarAgent.
Hopefully, some of this works for you.
Maybe I'll give this a try, however the Apple Support agent did have me save my calendars and then import them into a new iCloud and there was no issue with the CalendarAgent, only on my iCloud account.
When you deleted your other calendars did you restore your old calendar into it? I don't want to lose all my events.
I had the same problem... I tried all the things Apple said on http://support.apple.com/kb/TS3999? in the Troubleshooting Calendar on OS X Mountain Lion (iCal on OS X Lion) section...
Almost everything... I didn't reboot... But my iCloud calendar list was empty. I then decided to slay/murder/kill (take your pick) the process CalendarAgent, restarted Calendar.app and voila the calendars appeared and started to download from iCloud and now everything works as intended.