This idea probably isn't going to appeal to HN much. It's not very revolutionary. But there sort of seems to be a bit of a gap for this level of geographically-close, online communication.
I'm thinking of approaching the marketing side of it with small flyers in people's mailboxes, just keep it small, target a street at a time, so that there's a bit of localized inertia in the signups. It definitely has the chicken-and-egg problem (i.e. why would I sign up when no-one on my street has signed up).
If anyone's got any comments, ideas or feedback - then please shout out. There's no monetization strategy behind it (yet), and I very much like the idea of helping to facilitate the creation of little street communities.