The EFF is encouraging everyone to open up their wireless network. I think this could be great... I used to leave my network open, but was eventually cowed into putting a password on it. IIRC, this was due to the combination of my ISPs prohibition on sharing, and concerns about network security and being held responsible for other people's abuse of my connection.
I hope that their information page can address my concerns; it would be so nice to be able to do this.
One commenter on Slashdot advocates the use of parallel "guest" and private WiFi networks from your own router, which should allow you to maintain network security and limit the bandwidth available to the public (both to discourage leaching and perhaps make it harder for guests to do illegal things like download movies).
A community-driven network may be ideal... but until then.