In my complex, we have a small system of personal invitation. Whenever a new neighbor moves in (and happen to be close enough to be in the shared Wi-Fi range), we slip them a small note under their doors offering temporarily 'free' Wi-Fi so they can get their stuff set up and running as quickly as possible.
Because we change the password monthly, we suggest a small donation if they wish to continue using the shared service. If they choose to pay up, we simply add them to the e-mail list, and they receive the new password every month. It's a trust system that works fairly well, especially when everyone isn't heavy on the bandwidth.
What are your thoughts on sharing Wi-Fi with the neighbors?
(Image by Flickr member Shinzui licensed for use under Creative Commons)