The UN will try to mediate a tribal dispute in south Sudan. This dispute is in the oil-rich Abyei region, and could threaten the peace agreement that ended the SPLA's rebellion. That agreement made Abyei one of the three Sudanese "special districts"-- which meant that international negotiators would help resolve continuing political and tribal disputes in those areas. Abyei is scheduled to have its own special election in 2011, where the population will decide if it wants to join "the north" or "the south." In other words, the area is an ethnic and political fault line that also has oil.
The Dinka Ngok tribe's claim to the territory is disputed by the Misariyah tribe. An international commission ruling favored the Dinka Ngok's claim. The "Arabized" (and Muslim) Misariyah served in many of the "Muslim militias" that fought for the Khartoum government during the civil war. The Dinka Ngok are farmers (many are Christian or animist) and they supported the SPLA.
Could a dispute in Abyei reignite the civil war? Yes. Is it likely? The government and the southern rebels have reached an oil-revenue sharing agreement that will only work if there is peace. It's tough to determine if every one is really "tired of fighting" but that is a common statement in south Sudan. In the background lurks a re-settlement issue. Many Dinka fled the area because of militia attacks, and they are now returning home. Developmental aid from international sources may ease this problem, and give negotiators a chance to reach an agreement the Misariyah will accept.