While we do know that Microsoft wants to add a notification center to its Windows Phone OS, there isn't much clarity as to when that might happen. The arrival of the said feature, however, might be closer than we think. According to rumors, the GDR3 software update for Windows Phone 8 is now being tested at Nokia India. A "Notification screen" of some kind is part of that software build, if the claims of a supposed insider are to be trusted...
Source: http://www.phonearena.com/news/Windows-Phone-8-to-get-a-notification-center-with-GDR3-update_id46686
Source: http://www.phonearena.com/news/Windows-Phone-8-to-get-a-notification-center-with-GDR3-update_id46686