I think that if the other Civ would want to, you could trade them something/they could trade you something if they wanted single way open borders, so that one person would have them open, and the other wouldnt
also in Civ3 you could trade territory maps, I think that's also a good idea that should stil be included in Civ4
also in Civ3 you could trade territory maps, I think that's also a good idea that should stil be included in Civ4