The first of April is probably not the best day in the world to announce a Notification Centre for Windows Phone.
Nevertheless this is exactly what Liquid Daffodil did. Their Unification app pulls in notifications from 3rd party apps, but only works with developers who have signed up for the service (and therefore likely not with any big apps such as Facebook and Twitter).
There is however an incentive for smaller developers to sign up, with the app promoting other apps which support the service, so it may see more rapid growth than we expect.
The app is free and can be found in the Windows Phone Store here.