iPad pop-up notetaking To serve the same basic purpose as my question about split-screen apps:

Is there an app that allows me to quickly (in one touch or zero!) bring up a notepad/text editor while I'm in another app?

This would be immensely useful for things like taking notes on a PDF that I'm viewing.
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I realize that certain apps may include note taking functionality, but these do not suit the need for a universal way of taking notes, which TextEdit on my Mac does nicely.
You can easily get this by running any text editor (even Mail would do) in parallel to the main app and then use the new (iOS 5/iPad 2) four finger swipe (or a double-tab on the home button to get the list of applications) to switch between the app and the note.