So what's to be done? Several things really. First, as far as 802.11 goes, you should be investing in more advanced wireless security products as well as the use of smart cards, etc. WEP is easy to crack for the dedicated hacker who loads up a Linux laptop, and uses readily available tools. And since most firms using standard WEP are not likely to change WEP keys that often, WEP really is not adequate for preventing much more than casual usage attempts. For a look at the tools available, or perhaps to scare yourself silly as to how easy this might be, Google is your friend.
The use of WPA etc, make cracking 802.11 networks harder, but if you can avoid any RF signal from entering or leaving your site, you reduce if not emiminate the risks from the passer by attacker. A US firm, Force Field Wireless has several products aimed at helping you to reduce the RF emisions. Their DefendAir Radio Shield paint, or your own paint mixed with Paint Additive, reduces the RF transmission through any paintable surface. With a few coats, you get little or no useful RF emsssions through walls, ceilings, etc. This might be an ideal product for use in a board room - although remember that the RF spectrum that is eliminated includes cell phones! And an office with no cell phones ringing is not all bad.
For the even more paranoid, a UK firm, Glasslock has special glass to reduce the the risk of evedropping via the glass.
These things are not a particularly cheap way of doing things. The paint additive is US$34.95 enough to mix with 1 gallon of your own paint, or buy ready mixed paint at US$69.95/US Gallon (128 fl oz). But there are places and uses for these things. And besides, even if you aren't paranoid, they're probably still out there looking to get to you, your network and your data.