The north-south border was established in 1956, but the border line has not been demarcated. Over the course of the Comprehensive Peace Agreement, both parties have been working on this issue. To date, they have demarcated 80% of the border. The remaining 20% is the problem area where there are populations on both sides of the border. The work is in progress. We have encouraged the parties to complete this work before the referendum, because the part of the border that remains to be defined could clearly cause conflict. And yes, there are ethnic, historical and geographical criteria that enter into the border definition.
On October 19th, 2010. See this statement in context.