Microsoft has previously admitted it "ran out of time" on a Windows Phone notification center, but it looks like the company is testing early future versions of Windows Phone with this feature in place. A reddit user has posted a series of screenshots from a recently purchased Lumia 920 that appear to include an internal Windows Phone build. The software identifies itself as version 12084.WPMAIN(wpbldlab).20130509-1407, meaning it was compiled on May 9th. An early notification center can be seen in one of the screenshots, providing access to the typical Live Tile notifications.
There's also a redesigned calendar interface with weekly views, and the ability sort applications by name and frequency in the app list. Microsoft appears to be working on multitasking improvements too, with the ability to close apps in the multitasking view. The Reddit user claims to have purchased the phone from eBay, noting that it has a number of test applications starting with the letter Z. Microsoft regularly includes these apps with developer units, so it's not unusual to see such applications on early builds of software.