In the UK something like this exists already, in a limited form: British Telecom has a service called FON which anyone with one of their wifi routers can opt in to for free. When you opt in, you agree to share a limited amount of the bandwidth on your router; in return, you get the ability to connect through the router of anyone else who's opted in.

It doesn't always work quite as well as you might hope - connecting can be a bit of a pain sometimes - but it's a great idea. They've provided a real incentive for people to share their bandwidth.