Quite a nice article about wireless network theft on BBC News.
I don't think it's any real ethical dilemma, though. To be blunt, if you're still using an unsecured wireless network at home, you are either a) lazy, b) ignorant, c) a moron or d) all of these things, and you deserve to be stolen from.
If you're going to put a wireless network in your house or flat, learn how they fucking work FIRST. You wouldn't buy a car without knowledge of how to drive it, or refill the petrol tank, so why do people buy computers or routers without knowing how to use important aspects of their functionality? See option d) above.
The guy with the unsecured wireless network in my block of flats, however, needn't worry about anyone stealing bandwidth from his ISP. I'm going to hack his router and put on some lovely WPA encryption. Who knows, I might even give him the key too. Though I'm more likely to just change the network SSID to "TRYTHEFACTORYRESETSWITCHYOUMORON" and see how long it takes for him to figure it out...