Keyboard Acoustic Emanations (2004) [pdf] (rakesh.agrawal-family.com)

This is brilliant.

I'll bet most, if not all of the best sysadmins in the world would be in utter befuddlement if someone in their office was using a simple "idle" iphone as a keylogger. It's so out there that it'll make you think you're going crazy: the perfect crime.

I'm not sure this will works with today's quiet keyboards, looks like they used the noisy IBM-style keyboards for the setup.