Great review from Wired News: Stalking the Wild Wi-Fi Network about the new gadgets on the market for detecting WiFi signals.
Apparently, Wardrivers - people who drive around with a laptop detecting wireless networks to use - "can now palm a detector that not only spots Wi-Fi networks and details their signal strength, but reveals the name of each available network. Even better, the Digital Hotspotter is the first detector to reveal the most important piece of information of all: whether a network is encrypted."
Handy if you are desperate for online access - but really scary if you're the one with the wireless network where people are poaching the signal. I really like the terminology though - check out Wikipedia's definition of Wardrivers.