We recently carried out a survey which found that around 50% of people polled admitted to stealing Wi-Fi internet access. I'm going to briefly describe the darker side of Wi-Fi stealing and why it's more serious than simply using your bandwidth.
Wardriving has been around for several years. It basically involves driving around with a Wi-Fi enabled device looking for Wi-Fi networks. Once a network has been found its GPS co-ordinates can be uploaded to a website and anyone with a browser can find usable networks in their local area. Quite handy, especially if you are a hacker.
So, you are a hacker and now you have a network you can use, so what? In the past you might have had to use your own Internet connection from home, university or office and your aim would have been to find machines to take control of. You probably would have done this by running various scanning tools over a certain IP address range. If you were lucky you would have been able to find a machine running an exploitable service and that would be your entry method. Once you 'owned' that machine you would start the process again from the box you have just gained control over.
One of the reasons you would want to have several computers under your control is that it would make tracing you harder if you decided to try and break into NASA's site or wherever. The Wi-Fi hacker simply has to use your network, launch the same attack and not care about going via several machines. It will be the ISP's subscriber that will get a knock on the door when the FBI investigate.
To be fair, you would probably have to be pretty unlucky to have a hacker steal your Wi-Fi and start attacking someone who cared enough to prosecute, but just because it is unlikely doesn't mean you should ignore it. Follow the tips provided on the survey page as an absolute minimum and the hacker might simply decide to use the open network in the next street instead.