I came across this today. If you're on wifi, click the Mapper, then click the button to have it find you. If Google's ever been in your neighborhood, it will. Now I know why they were scraping SSIDs and MAC addresses. Felt a bit of a chill. http://meraki.com/tools/mapper/
Google has definately been in my area and the Mapper couldn't find us. Our WiFi is locked up pretty well though.
Of course if you've got an iPhone then it's pretty easy to find out where you've been.
You may not know it, but a wireless router constantly broadcasts the following unencrypted information, regardless of the encryption method used. Not specifying a SSID doesn't "cloak" the router: