From the US standpoint: I still adhere to the principle that the airwaves are public (regulated to prevent interference, but still public). Anything broadcast should be fair game to any citizen that has the ability and knowledge to receive and translate it (don't get me started on my state's ban on radar detectors...).

The wifi routers and network cards are all approved for use by the FCC. How they get used beyond that approval is up to the owners. Some people like to leave their wifi access open, either out of kindness or for plausible deniability purposes. Others are ignorant, but that's no real excuse. And then there are those that lock down their access points as tight as possible.

I had my connection open for a while, then some neighbor started chewing up obnoxious amounts of my bandwidth (must have been downloading movies and such, it got really slow). So I fixed it. Turned on the best encryption offered and added MAC filtering. If you don't want your connection used, take the antenna off your router and put a terminator on the connector. Or at least turn on some form of encryption. Plug your PCs into the router with Cat5. Don't buy cordless phones, don't use cell phones. Otherwise don't be too surprised if your transmissions are intercepted or manipulated.