Expanding the widgets and capabilities of Notification Center is a favorite hobby of jailbreak developers. Prolific note takers might like the recent jailbreak tweak NCMemo, which is available for free on Cydia. NCMemo inserts a text box directly into the Today tab of the iOS Notification Center.
Once installed, NCMemo can be switched on under Notification Center -> NCMemo. The text box can be resized and moved directly within the Today tab. Colors and fonts appearing within the text box are fully customizable under Settings -> NCMemo. To create a note, just open the Notification Center from anywhere in iOS and enter text.