Ah, I do know all my neighbours, but I live in an odd sort of cul-de-sac. And growing up, we didn't know all our neighbours.
It's true that kids aren't generally allowed to play out any more. My friend with the 9-year-old doesn't let her kids play out, but then, in her street, all the back gardens are joined up, so the kids can all play safely there, without access to the street.
In Montreal, they still have back alleys, and kids are still allowed to play out in the alleys. Their moms can generally hear or see them from their kitchens, though, at the backs of their houses. It's a nice arrangement.
My eldest son is 4. I doubt he'll get to play out on his own until he's 10 or something, and that seems a bit sad to me. But then, there never seem to be any kids playing out on their own around here, anyway.