It is unfortunately illegal to make use of the network if it is not advertized for public use, even if it finds you
There are a lot of places that do have public networks though like coffee houses etc. The thing is, I don't think you would really have any issue ever as long as you are not being malicious and just using it as a connection etc. Any admin with that much interest in protecting there network would attenuate it properly and protect it with some security to begin with. If they aren't doing that, there is not much chance they are interested in logging your packets to figure out who you are to prosecute you.
On the project idea, there are several cities already doing this. Check
Seattle Wireless