An article published by Dreamlab Technologies on The Register talks about how researchers have cracked the encryption used possibly by most non-bluetooth Microsoft wireless keyboards.  There is even a video that shows in real time as they see your keystrokes while you type. 

Dreamlab cracked the encryption key used within Microsoft Wireless Optical Desktop 1000 and 2000 keyboards. As most products in Microsoft's wireless range are based on the same technology other products are likely to be insecure. Max Moser and Phillipp Schrödel of Dreamlab Technologies succeeded in eavesdropping traffic from a distance of up to ten meters using a simple radio receiver. More sensitive receivers may make it possible to capture keystrokes over larger distances.

I use a wireless mouse but not a keyboard but even then I only use Bluetooth.  Fortunately for everyone Dreamlab's has refused to release their methods and instead sent the information to Microsoft.