I'll answer this question in two parts. One is on the process of negotiation between north and south over Abyei. This enclave has been subject to an international law case in the Hague that in effect awarded, for the interim period, most of Abyei to the south, but the ultimate status of it will be determined in the referendum.
Unlike the southern Sudan referendum, for which preparations are under way, even if behind schedule, preparations have not begun for the Abyei referendum. It's because of this issue of north and south not being in agreement on voter eligibility. Negotiations have taken place in several rounds, one near New York City two weeks ago, and then in Addis Ababa about a week ago. Another meeting is supposed to take place in Addis Ababa. Up until now they have not been successful, so there's a deadlock, which means there's a stalemate. Meanwhile, tensions are mounting. There was a serious shooting incident a couple of days ago.
The status of the Misseriya, this nomadic tribe, is perhaps the biggest issue at play. The settled population are southern-identified. They're the Dinka. The Misseriya, which have always migrated in and out of Abyei, are northern-identified and Arab-speaking. The question is, how many of them can be considered residents of Abyei for voting purposes? They simply haven't come to a conclusion on this, and it becomes extremely sensitive and perhaps inflammatory.