If I remember correctly, notification centre will be for MAS apps only, meaning using it in QS will be a no-go.
See the second last paragraph of this article on AnandTech:
There's been a bit of confusion about the new notifications API (NSUserNotification) and whether or not it's available to applications not in the Mac App Store. Local notifications using the new API are available to third party apps regardless of their distribution model (they don't need to be in the Mac App Store)...
Should just be push notifications that are MAS-only.
Also I'm currently using Now Playing (via TUAW) to show my currently playing iTunes track via Notification Centre, and I downloaded that from MediaFire (not MAS).
To update everyone: The current plan is to get rid of Quicksilver's built-in notifier and send those things to Notification Center instead. Growl will continue to be an option as it is now. The original plan to go Growl-only was scrapped, as it makes it difficult to tell Quicksilver sent the notification.