Photographer Thierry Cohen has put together a number of images of what the sky above major world cities would look like if the cities had no lights. The takeaway: city dwellers would see a lot of stars. Here is one example:

My first thought is that such scenes would only be possible in reality in a post-apocalyptic scenario (which is quite popular these days). But perhaps a city could undergo a project like this for a night or weekend? Imagine a positive sort of Carmageddon but not quite Earth Hour which is more about reducing energy use and environmentalism.