When analyzing Wi-Fi networks in particular, we have to take the borderless nature of radio signals into account. For example, someone could be parked in a car outside your house, running a rogue access point and tricking the computers inside your home to send all their traffic through this nefarious surveillance equipment. To be able to detect such attacks, you need a way of monitoring the airspace around your house.
Kismet is a Wi-Fi spectrum and traffic analyzer that relies on your Wi-Fi adapter's ability to enter something called monitor mode. You should be aware that not all adapters and drivers support this mode of operation. Your best bet is to look for an adapter based on the Atheros chipset, but Kismet will try to detect and use any adapter—just give yours a try and let others know about it on the Raspberry Pi forums (http://www.raspberrypi.org/forums/).