It has been learnt that third party widgets in the iOS 5 notification center are possible. As of now, Apple has only included widgets of its Stocks and Weather apps in the notification center, and does not give any option to add third party widgets. However, there might be a way to get third party widgets in the notification center.
While it is possible that Apple may only allow third party widgets to display limited information or have limited functionality, one must not forget that many developers choose the way of the jailbreak when developing their apps and tweaks. So it is highly likely that we might see third party widgets for iOS 5 notification center release in Cydia, as it is now possible to jailbreak iOS 5 beta 1 tethered on Mac.
We'll be sure to let you know once third party widgets are available in Cydia. Keep watching this blog.