A WiFi detector shirt does exactly what you wanted: it detects Wi-Fi signals and shares it with the world through ... your chest. Detects both 802.11b and 802.11g, and the signal strength is displayed in real time (see picture). After you stop broadcasting yourself to the world (a new kind of super hero?), Pull out the batteries and wash away the sweat of hard work, after a whole day of Internet theft :)
The Wi-Fi Detector T-shirt is available exclusively on Think Geek and will be on sale from October for $ 29.99.