There was a post on c|net about how stealing Wi-Fi in an airport is bad. I agreed with the following comment.
I agree, this sort of unauthorized use of Wi-Fi on a continuing basis is theft. Quite clearly the service is made available for the purpose of making a profit, and so circumventing the system constitutes stealing. Kind of like phone phreaking; while it's cool to figure the ins and outs of the technology, going much further past demos is hard to justify. This thing called ethics says just because you can doesn't mean you should.
Now I would change my reaction if we're talking about an unprotected signal coming into my house. It's my airspace, and if someone is broadcasting something I don't want at me, then it's their problem if I decide to take advantage of it. The law is dead wrong in saying that that is theft. If I chuck an iPod into someone's open window and they decide to keep it, it's not theft since I put it there. (Actually, it's my bad for throwing stuff into their space, and they'd have a right to be upset.) Decoding a signal is more marginal ethically. I'd say it's "half theft" in that as above, the user is stealing service, but it's also half the broadcaster's fault for polluting the PUBLIC (outdoor) airspace with their private signal, in contrast to the private airspace within the airport. With publicly shared things like frequency spectrum, there are gray areas abound.