There's a wonderful little (freeware) program called "NetStumbler" which will convert your wireless laptop into a WiFi scanner. I've used this and gone for a drive round the local area with very interesting results. About 25-30% of all WiFi connections have NO security at all, and more worryingly some folk change the SSID to their name or house number!!!! Why not just put a big illuminated sign on the roof saying "I've got a computer, please break in"
I suspect that many customers get their "free wireless router" and have no idea that it will be enabled by default, thereby making their internet connection available to all. I have WPA and MAC code addressing setup, I also turn the router off at night or when it's not being used for a while.