> Does anybody know of any good reasons why the clipboard shouldn't be secure?

Seems like a UI issue to me. Right now apps themselves invoke the paste command (e.g. they can put a "paste" button in their UI). If you remove clipboard access from programs, the UI would have to be presented by the OS.

Requiring paste to be initiated by the OS may work alright for text boxes (but you'd now have to sandbox all the text boxes to avoid the app simulating taps), but it wouldn't work for say, pasting images.

Another option is to add a dialog box every time asking the user "did you REALLY want to paste?", but that will quickly get annoying.