Microsoft is working on an update to Windows Phone 8 is now to be called Apollo +. The plan update will be minor version updates to add features that need to be added to the main release, such as VPN and network support OTA updates.
It will also include bug fixes from issues such as bugs Wifi connection. So to say the release of Windows Phone 8 is currently still leaves important features, such as notification center, and will come in the next update of Windows Phone 8.
Microsoft plans will provide details on this update at Mobile World Congress in February 2013.