Device called KeySweeper, which is developed using Arduino board and camouflaged as a functioning USB wall charger, can wirelessly and passively sniffs, decrypts, logs and reports back (over GSM) all keystrokes from any Microsoft wireless keyboard in the vicinity.
As per its developer SamyKamkar :
All keystrokes are logged online and locally. SMS alerts are sent upon trigger words, usernames or URLs, exposing passwords. If unplugged, KeySweeper continues to operate using its internal battery and auto-recharges upon repowering. A web based tool allows live keystroke monitoring.
He also posted a photograph of the device and a video explaining its operation.
Opensource code and schematic is available at Github page https://github.com/samyk/keysweeper . Happy Hacking :).