Here's a short radio broadcast about Fallen Fruit, an organization that collects fruit on public land in Los Angeles (mostly from private trees overhanging public land) and more importantly, is making maps of existing fruit trees and lobbying the LA government (unsuccessfully) to plant more fruit trees.
The city notes that rats, high water needs, and the danger of "fruit fall," are reasons why it won't plant fruit trees on public land. This, I admit makes sense, but Fallen Fruit's point about creating free urban food sources also makes sense. Perhaps the middle ground is finding trees that provide food without the danger. In some cities, that could mean sugar maples, in DC, mulberries could play that role (and do to some extent). I'm sure rats eat some of the berries, but there's no danger from fruit fall (mulberries are soft) and DC is pretty wet so water use is probably not a concern.
But why stop there? Fruit doesn't just grow on trees. Why not get cities to plant blueberries and blackberries along the edges of land--it'd make good natural fencing and provide fruit (without the danger of fruit fall).
Fallen Fruit is also encouraging others to map their cities' fruit production--take a look. And then, if you have time, make a map.