re: Mountain Lion adding default alerts that you can't get off you iPhone/iPad
Found a workaround that fixed the problem for me. All posts and other help did not work on my MacBook Pro/iPhone 4s/iPad 3....Here's what I did. First, exported each individual calendar to a .ics file on my desktop. Then removed all calendars. Then went to Calendar->Preferences->Alerts and selected "None, None, None". The check box for "use these default settings only on this computer" is checked and grayed out, so I could not change it. Then went to System Preferences->iCloud and unchecked Calendars and Reminders, and Documents and Data. Got the warning that it would delete data on MacBook but that's the only option, so I did it and did not seem to lose any data. Then opened iCal and imported each .ics file from the desktop. After that was done, I checked Calendar->Preferences->Alerts again and it had changed back to default alert at 9:00am, so I changed again to "None" . Then I went to iPhone and iPad and in Settings->Mail, Contacts, Calendars, changed Calendars->Default Alert Times-> to "None, None, None". Then on iPhone and iPad went to Settings->iCloud deselected same Calendars and Reminders, and Documents and Data. Restarted all...sync'd to iTunes, first time checking replace calendar data on iPhone and iPad (Bottom of "Info" tab)...Then sync'd again and all alerts are gone now on all devices and everything is working like it was using Snow Leopard. Takes a few steps and a little time, but this worked great for me and no issues since.
One thing to mention is that I tried all these steps without deselecting "Documents and Data" in iCloud settings, and it didn't work. Had to do each step in above order to get things the way they were before Mountain Lion.