There are Christians in the north and Muslims in the south, and one of the key concerns around the referendum and independence is respect for citizenship rights and what that means. A lot of the southerners went back to South Sudan out of sheer fear, but there are still some left in the north who have grown up there, who were born there, and who are committed to staying. Before all the parties pulled out, some of them ran, or tried to run, in the previous elections.
The region of Abyei I don't quite understand fully. I know it's heavily armed. There are a lot of armed personnel there, and there's a big chance of it exploding, just by human error or ill will, and there have been increased conflicts. The issue of citizenship rights is a very important one.