I prefer the cultural borders over the civ2\1 approach of just that cities actual tiles. I mean one settler could rape you if there is even one open spot(which there always was) in your massive civ. I like the idea of cultural borders but would prefer some sort of set political borders, however how to decide these set borders would be a bit of an issue. Perhaps one could send a unit to every square outside of your city radius to claim. Obviously there would need to be some sort of limit on how far away from ones city you could claim. Perhaps 2 squares in every direction? That way it would require you to keep cities at least somewhat close to one another. Of course when one hits another's civ's territory neither could claim what has already been claimed by each other, with the exception of war and then still only when within two squares of max city radius.