The New York Times files an amusing story about neighbors and travelers using open home Wi-Fi networks: The article is dead in terms of people's attitudes. Most people don't want other people on their networks, but those that leave their networks open also don't want to hassle with securing them. Buffalo has been offering one-button security through their AOSS system for a while, and Atheros and Broadcom each have simple, robust methods of adding Wi-Fi encryption without inventing one's own long passwords and manually entering them. The Wi-Fi Alliance told me in January that they expect to have a unified proposal later this year.
The article notes that users accessing people's networks could snoop or carry out malicious activities. Worse, however, is that a local network is usually given less scrutiny by firewalls and thus a user who piggybacks onto your network and whose machine is infected with viruses and worms could unintentionally compromise your systems. That's a bigger risk, in my view.
The Brodeurs, featured in the opening of the article, note at the end that after adding encryption to their network, neighbors who wanted access offered to pay. They demurred. It's a reasonable decision. Even if you run strong firewalls or use something like Buffalo's privacy separator feature (which keeps each user's data on separate virtual segments), other users can still sniff your traffic as it passes through the air, and can still burn up your bandwidth.
While Speakeasy offers network sharing and features for them to bill your neighbors, there's a missing piece: a piece of hardware that would provide built-in WPA Enterprise (for unique encryption keys for each user), network separation, and bandwidth throttling per user with revocable credentials or passwords. Elements of this exist in a number of gateways, but there's no way I know of to buy a commodity, inexpensive gateway that would combine all this.