The team at Remote-Exploit.org are currently working on an open source hardware/software sniffer project that allows you to decode Microsoft wireless keyboards (and Logitech boards in the near future).
According to the project page, Keykeriki is intended to enable "every person to verify the security level of their own keyboard transmissions, and/or demonstrate the sniffing attacks (for educational purpose only)". Yes, "educational purposes." In case you don't know, keyboard sniffers allow the user to eavesdrop on what is being typed by analysing the electromagnetic signals produced with each keystroke.
At this point, the software and schematics are available to download from the site, but a pre-made board is not ready for release just yet. However, they should arrive soon along with add on modules that include an LCD display and an interface that works with your iPhone. Sigh...I'm sticking with wired keyboards for now. [Remote-Exploit via Hack-a-Day]