From what I have heard in interviews, closed borders are just that-CLOSED!!! Other Civs cannot send troops, settlers missionaries etc etc AT ALL-unless they declare war with you. However, if you wish to reap the full benefits of trade, apparently you do need to have borders open to that particular nation. So this would suggest very strongly that you can be selective about with WHOM you have an Open Border agreement.
Beyond that, I know no more than you guys!!!
Yours,
Aussie_Lurker.