On the outside, the KeySweeper looks like a regular wall wart, but on the inside is an Arduino microcontroller that allows the KeySweeper to connect to nearby wireless keyboards and passively sniff, decrypt and log all of the keystrokes. These keystrokes can then be transmitted over the Internet by a wireless connection by using an optional GSM cell phone module, or stored on flash memory inside the device.
You can read more about the security risks of the KeySweeper in an article by Adrian Kingsley-Hughes at http://goo.gl/wbj64e while the KeySweeper itself is described in detail at http://samy.pl/keysweeper/.
Please note that the KeySweeper is not being offered for sale. It is a “proof of concept” only. However, anyone with electronics ability can probably duplicate it. I suspect that someone will be offering similar units for sale within a few months.
NOTE: This article was written by using a Macintosh wireless keyboard. I just reached into the closet and pulled out my older wired Mac keyboard and will swap the two momentarily. You might consider doing the same.