The screenshots were shared by a Redditor who bought a 920 off Ebay and found that it was running a peculiar OS: an internal beta. In addition to the notification center, there seem to be some improvements to multitasking as well, including the ability to close applications in that view.
As with any leak, there's always the possibility it's just a big fake, but there's nothing about this set of screenshots to make it wildly suspicious. On top of that, we know Microsoft's pushing out a Windows 8.1 update soon, and when an update comes to Windows Phone 8, you'd hope it would include something so basic as a notification center. Here's to hoping. More here.