Replicate the ios screenshot effect

This may get you started:

http://stackoverflow.com/questions/4239692/draw-on-directly-above-desktop

Use a borderless NSWindow that fills the screen, and an embedded NSView (background color set to white) that fills the window.

Bring the window to the front/focus, then use core animation to fade it out. (And eventually close it. Don't forget to close it!)

 

I'm not certain that its possible to receive any notification that a screenshot is taking place... Unless maybe you can moniter:
1 - the directory in finder where screenshots are kept (which you'd have to find from a preference plist hidden in library, no doubt)
2 - the user's clipboard for changes in the image data stored?

I do not believe you have any chance of an app with these features being sandboxable.