The South African Constitutional Court (at the same level as the U.S. Supreme Court), has ruled that registered voters living overseas can vote for the National Assembly in the South African elections to be held on 22 April 2009. The only proviso is that they must notify the chief electoral officer that they intend to vote by 27 March 2009.
You can read about it at News24.
22 April: South African election date
If you, as an expat (or any South African for that matter), would like to know if you are registered as a voter, simply go to the IEC website and enter your South African ID number to verify whether you are or not. If you are registered, download the VEC10 form, fill it in and send it back to the IEC before 27 March 2009.