Excellent report on the state of Wi-Fi data collection and how it's continuing to expand: Google doesn't collect Wi-Fi from Street View (at least in some countries) following its data acquisition debacle, but Android does. And iPhones. And trucks driven by Skyhook Wireless. And other sources.
Bob McMillan at IDG News Service runs through how it works, the current efforts, and where privacy concerns lie. In general, publicly broadcast information is hard to contain, but McMillan examines the connection between collecting millions of SSID and MAC association by location and making this available in easily retrievable form, as Google does.