Techdirt has a post on wallpaper that is designed to block WiFi signal. I was actually thinking about something in the same vein the other day as I was sitting outside my house with my laptop, using a network stumbler to discover almost 100 wireless access points in my neighborhood (yes, very few of them had the security enabled).
I'm sure, as Techdirt also supposes, that this material is going to be fairly expensive. I initially wondered if this would be broad-spectrum blocking material (i.e., your cell phone and other radios would not work) or selective, and when I clicked through to the source article, I found that the material is called "Frequency Selective Surface," meaning that it can specifically target a discreet frequency range. 2.4ghz is the range of WiFi, so I was trying to think of anything else that operated in that range that I used at the office -- I couldn't think of anything.
The material is available in both active and passive varieties, which brings up an interesting thought: If you installed active panels in new construction that you had the ability to turn on and off, and had those panels tuned to block different cellular service provider frequency ranges, in the event of a terrorist attack, you could kill off all cellular service within a building, but still be able to use public service radios on alternate frequencies. I wonder if the FCC cell phone jamming laws cover active surface materials.