A recent discovery in an Apple patent application suggests that third-party widgets may be planned for the Notification Centre in iOS 5.
In it’s current form, iOS 5 does not support third-party widgets but the following image posted by RazorianFly would seem to indicate that the potential for Notification Centre widgets has been built in to the OS architecture, with parameters “User-Created Widget(s)” and “Widget Creator Module” clearly visible in the above image.
Technically, these kind of widgets are already possible, jailbreakers have shown with the jailbroken iOS 5 beta:
Unofficial iOS 5 Notification Centre Widgets: SpringPrefs [left] & Music Center [right]