Ars Technica:

French researchers at Institut Polytechnique de Grenoble, in cooperation with the Centre Technique du Papier, have developed a wallpaper that can block WiFi signals, preventing them from being broadcast beyond the confines of an office or apartment.  But unlike other signal-blocking technologies based on the Faraday cage (which block all electromagnetic radiation), the wallpaper only blocks a select set of frequencies used by wireless LANs, and allows cellular phones and other radio waves through.

Apparently, you’ll be able to use a classic wallpaper on top of it for a better look.

I hope anti-wifi carpets and ceiling paint are in progress.