Impossible to predict. Those of us here are expecting to return to work after the Xmas break, but we've made plans to get out at a moments notice. It's a day by day situation for the foreseeable future (measured in months rather than days) which possibly could deteriorate to an hour by hour senario.
If you're going Gedaref, KHT, Wadi Halfa you should be OK safetywise. The only caveat is that the northern authorities may prevent your onwards travel at any point, especially during the referendum time.
Do not attempt to cross the borders between North and South Sudan around that time. There has already been shelling along the border in Abyei State last week.
The forecast is also that we can expect increased JEM rebel activity in the North & the same for LRA in the South. Although these groups are not targeting foreigners, both have a history of kidnappings.