Voting is taking place in the referendum on Southern Sudanese independence. The text of the Referendum Act is available at: http://www.ssrc.sd/SSRC2/newsphoto/en-act.pdf
Section 25 deals with voter qualifications. There is no requirement of Sudanese citizenship; rather, there are two tests of "residency": (a) born to parents both or either of whom belong to an indigenous community of Sudan before September 1, 1956, or has ancestry traceable to one of these communities; or (2) has permanently resided without interruption, or has parents or grandparents permanently residing without interruption in southern Sudan since September 1, 1956. (September 1, 1956 was the date of Sudanese independence). This is one of the longest residency requirements I have come across in an independence referendum (55 years).