The Ethicist endorses borrowing a neighbor's Wi-Fi signal: In a fairly one-sided debate of the issues, the mention of Time-Warner's Roadrunner threat letters to purposeful Wi-Fi sharers aside, The New York Times's columnist Randy Cohen says that unless you inconvenience the unintentional service provider you're borrowing from, you're not going to ethical heck.
His summary of Time-Warner's issue is specious, though. The company argues, in effect, that while you may have a glass of water at a neighbor's, you may not run a pipe from his place to yours. Actually, because the service is unmetered, it's more like saying, we're providing you unlimited water for personal use, and guests are okay, but you can't run a pipe to a neighbor's house.
(Cohen quotes Mike Godwin, the formulator of Godwin's Law, which is infallibly accurate.)
(I like the sound of "unintentional provider." I've been trying to find a term to cover the difference between community wireless nodes run by individuals who aren't necessarily bound to keep them running and community wireless nodes and other free nodes that are designed and "advertised" as available all the time. I was thinking purposely persistent provider, but perhaps the distinction is "unintentional provider" and "intentional provider.") [Nods to Cory D. for prompting this digression.]