Such a device can be taken to a war, or disaster, zone, and give someone access to what's going on in most wi-fi networks. These wireless nets are very popular because they can be quickly set up, and instantly give a large number of users access to one satellite, or ground based, Internet link. In Iraq, even terrorist and warlord groups have been found using wi-fi. Using stronger encryption slows down user access, and most avoid it. But if sharing your information could cause trouble, be aware that the capability is out there, to share that data whether you want it shared or not.
Wi-fi networks continue to grow in popularity among military and disaster relief organizations. At the same time, the vulnerability of these networks is increasing. For example, hackers have demonstrated a hand made device, about the same size as a suit case, that can monitor, and grab information, off up to 300 networks at once, and over a wide area. This "Janus Computer", allows captured data to be saved to a hard drive. Even if wi-fi networks are protected by encryption, like WEP, the multiple processors in the system can crack it in almost real time.