Some improvement will come if the referendum happens and the results are accepted - then they need to fix the situation in Darfur for further improvement.
U.S. Sen. John Kerry says Khartoum will win quick U.S. incentives if an independence referendum in the south goes smoothly, but further improvement of ties depends on progress toward peace in the separate conflict in Darfur
http://www.ajc.com/news/nation-world/kerry-darfur-integral-to-798035.html(this is just 4 paragraphs long, so I was not sure what I could quote.