Certainly the most contentious component of the North-South peace is the dispute over the boundary. The disposition of the town of Abyei - and on who's side of the boundary it lies - compresses the contention into a smaller and more volatile piece of real estate. Certainly that Abyei and its environs hold oil reserves, and that both sides have ratcheted up the "never-back-down" rhetoric, hasn't added anything but fuel to this fire.
The Comprehensive Peace Agreement of 2005 created a process to address the North-South boundary delineation and the status of Abyei. For a number of reasons (lack of a strong Assessment and Evaluation Committee, focused outside parties, keeping pressure on both parties, personalities, etc.), the CPA-detailed path didn't achieve the outcomes it was designed to; the North rejected the process when it was determined not in their favor.
Fortunately, both parties agreed to take the matter to the Hague for binding arbitration in mid-2008. This single act might be recognized as the most significant step in resolving this flashpoint issue, and thus maintaining the North-South peace. I don't think this can be understated.
Read more about it here.