well its already been stated that higher cultures will eventually creep onto enemy borders, and perhaps even ENGULF enemy cities.
and there are a ton of border threads, dolt .
anyway, i do like your idea of borders being influenced by geographical terrain, though i believe it has not been implimented.
i remember a huge debate in a screenshot, a mountain or hill was in a city's border.
not sure if that means anything, btu i think geography plays a minute role, if any at all.