NORTH SUDAN:

Assuming your home mobile operator has a roaming agreement with one of the networks in Sudan, GSM mobile phones work in major towns in Sudan (Khartoum, Dongola, Gedaref, Port Sudan etc). GPRS doesn't work. You can't rely on mobile phones in the desert (obviously) - take a satphone if you're heading somewhere remote and alone.

SOUTH SUDAN:

there are plenty of mobile operators that works in varying degrees in the different south Sudanese towns. Generally, the mobile operators from Kenya and Uganda also work well in South Sudan. As does European and U.S. operators that offer roaming services. But for Blackberries, the extra services (e-mail etc) are all blocked due to the U.S. Embargo on Sudan. iPhones work well!  Satphones are good in the bush and driving from A to B, but now most of the major towns have mobile networks.