Tomorrow (9th July) we will have a new country - The Republic of South Sudan
The Guardian has conveniently given us a new PDF map of the world for download
You should update your databases / lookups to reflect the change.
There is no official ISO-3166 code as yet, but it is likely to be SS.
If you haven't already done so, you should also have the most recent new country codes:
XK - Kosovo (2008 temporary)
ME - Montenegro (2006)
TL - East Timor (2002)
You can download a full list of ISO country codes from here - although it doesn't include South Sudan or Kosovo