My Windows 7 64 bit Outlook 2010 pop3 usb calendar sync also stopped working after update to ios 6.1.2. Never had a problem with it before. All other syncs work fine. iTunes displays calendar sync step clearly in progress window with no errors. Here's what I did:
-Completely deinstalled and reinstalled iTunes and related components per Apple specs.
-Disabled all non-Apple com addins in Outlook.
-Ran scanpst against .pst file to make sure it was clean.
-Made sure Outlook version was current and that installation was clean.
-Pushed calendar data from PC to iPhone using both the "do not sync items older than 30 days" option and no option.
-Deleted calendar from phone in iTunes.
-Completely reimaged phone.
-Repeated above steps several times.
-Completely repeated all above steps on a different PC running Vista 64 bit with Outlook 2003 and the same .pst file with exactly the same results, ie no calendar sync.
-Took the second PC to the local Apple Store where the techs repeated most of the above steps with exactly the same results. Techs were very professional, but were not able to make any headway.
-I asked if they could backdate the phone to a previous ios and they said it was not possible.
I have no doubt that the problem was introduced with the update to ios 6.1.2, and had I known about it I would not have updated.