If your auto-sync was suddenly disabled after upgrade to CyanogenMod 10.1 (in my case that is the beta version) and you can see the annoying "Sync is OFF" message under you Google account settings, there is a simple way to re-enable it. Go to Settings->Data usage and press your phone's menu button. Select the "auto-sync data" option and that's it! It took me a while to figure it out as the placement of this option is completely counterintuitive.