Right now we're defining localities as counties and cities. Two Marysville hopefully would have different zip codes or some other way to tell them apart.
We're not really looking for street maps. We want to display membership changes of our local groups. On the local level we want to be able to display membership by zip. I hope this application will have other uses beyond this.
I haven't come across voting software that seems useful yet. We wouldn't mind using the voting precint as the basic unit. Even if there was something, I'd rather 'roll my own' since this seems like an interesting project to me.