If there is open wifi in public, I am going to use it. With so many free public wifi available, it is easy to argue that I thought if was a free network.
I think legal issues come down to intent. If someone installs a directional high gain antenna on their house to borrow wifi, well that is obviously wrong. But if I am using a standard device (iphone, laptop, etc) and I can pick up a signal in my house or in public, I see no issue in using it.