I've got an iPhone 5, Macbook Air and iMac, both bought in late 2010 and running Mountain Lion. When adding a bookmark to Safari, iCloud is supposed to sync it across all platforms, but it only works on the Air and iPhone. On the iMac I have to go into settings -> iCloud and uncheck the Safari button and then check it again, after a minute or so the bookmark then gets synced. Has anyone else experienced this and found a solution? Might it be fixed in a future update?