I just discovered the issue this morning, but Calendars, iMessage, and Reminders are not syncing; Notes, however, are (I haven't tested Contacts, and I don't use Mail).
I entered an appointment for today in iCal on my Mac (Mountain Lion 10.8.2) on Tuesday. I checked my iPhone (iOS 6) to see when the appointment was, and there was nothing there. I checked iCal, the appointment was still there. Problem is, it never made it's way to iCloud.com.
I ended up deleting my iCloud account from my Mac and reinitializing it, but nothing.
I created the appointment in iCloud, but it never synced to my Mac or my iPhone. Deleted it, then created it on my iPhone. Again, nothing.
I checked iMessages, and here's a bit of an oddity: I sent myself a test message from the Mac, which I did appear on my iPhone. Cool. Awesome. I sent myself another message from my Mac a few minutes later, and nothing. However, in the list of messages on my iPhone, it shows the timestamp of the test message as when it was sent from my Mac, but when I went to view the messages on my iPhone, it still showed only the first test message. I sent another test, this time from my iPhone, and it arrived on my Mac. Another test from my Mac, and it came thru on my iPhone.
During this debacle, I was texting my girlfriend, but our conversation history during that time did not appear. The last message that shows up in iCloud is from this morning just past 9.
After reinitializing iCloud on my Mac, my Notes all appeared on my Mac. Wunderbar.
None of my Reminders made it back to my Mac, and after creating a test one, it never showed up.
Does anyone have any idea why there is so much intermittence between iCloud, my Mac, and my iPhone?