The Enough Project published in January 2012 a useful survey of Sudan and South Sudan's relations with their neighbors: Egypt, Libya, Chad, Eritrea, DRC, CAR, Ethiopia, Uganda and Kenya. Written by Omer Ismail and Annette LaRocco, the brief report describes the state of relations between these two countries and the impact of neighbors on issues such as borders, oil and political stability.
You can read the report here.