CNet has a great FAQ-style article on the current kerfuffle surrounding WiFi signal poaching. It's worth a read if you have a WiFi network in your home or business. There are two points worth considering. Most service contracts from DSL and cable providers prohibit sharing your bandwidth with other locations. So if you keep an open access point so the little old lady across the street can download some songs from iTunes once in a while, it's most likely a violation of your service agreement.
Second, an open access point can be used for illegal activities. While you are not likely to be held liable, open access points are a target of some spammers, who hijack the signal to dump a few million emails on the network (which does not take as long as you might think). Other unpleasant uses include using a "borrrowed" access point to download illicit material like child pornography. Using someone else's Internet access makes it much harder to track down such activities.