The way it works is rather simple: you can compile a custom widget by creating a class that implements the BBWeeAppController protocol which allows you to have the widget launch local apps when tapped, manipulate its height, set rotation detection, etc. This can work with stock iOS apps as well as App Store apps, with custom widget bundles being stored in the /System/Library/WeeAppPlugins/ directory.
As a basic example, you could create a custom widget to list recently created notes in a snippet view, and when a note is tapped it will take you directly to the Notes app. In theory, there's also the possibility of nesting WiFi, 3G, bluetooth and other toggles in the Notification Center (similar to SBSettings).
Apple currently has weather and stock widgets available in the iOS 5 beta but hasn't mentioned anything about extending a widget API to 3rd party developers, so this is a good sign the jailbreak community may come to the rescue by allowing additional functionality within the Notification Center.
Are you looking forward to custom Notification Center widgets in iOS 5? Let us know what you think in the comments!