If I create a new note on my iMac (OS X 10.8 Mountain Lion), it will not appear on either iCloud or my iPhone 5 (updated with the latest iOS). However, I CAN edit existing notes and those changes do sync to the cloud and to my iPhone. Very Annoying. I can create a new note on either cloud or iPhone and it all syncs. I just CANNOT create a new note on the Mac and have it sync to the cloud.
Also, in case you wonder, I do not have this problem with either calendar entries or contacts. I am able to create new contacts and events on my Mac and they all sync to the cloud and my iPhone.