A Florida man has been arrested and charged with theft of a WiFi signal. Ben Smith was apparently parking outside someone's home regularly to "borrow" the broadband signal.
This may sound inoccuous, but suppose someone stopped by your flower garden every day and cut a few of your roses for their own use? Or if they walked into your yard twice a week in the winter and took wood off your woodpile?
Much has been made of "wardriving," which is for some a kind of sport--driving around looking for open networks to get free broadband access. One might argue that if households are using WiFi without access control or encryption, it's their problem, but that's akin to saying that it is okay to steal a TV from someone's home if the door is left unlocked.
A friend of mine who moved recently was able to connect to four of his neighbor's WiFi signals (he has his own service, so he is not "borrowing" from the neighbors). It's just good policy to take a few minutes to turn on encryption and access control in your wireless router. And it keeps your data files from prying eyes (an "open" WiFi router will often let anyone who picks up the signal get full access to the contents of your hard drive.