Hi
Yes, IB is offered in conjunction with the American diploma. The IB is accepted at the best universities of South Africa and generally meets the requirements of the National Matriculation Certificate (which is largely dependent upon which examination board it was from in the country). Private examination boards are of a similar standard to the IB (less the CAS/TOK/EE).
Six subjects are taken over a similar period and lead to final examinations which are externally set and externally examined. They must include maths, two languages (one of which must include Afrikaans (by law) unless you are exempt because you are not a South African national).
The University of Cape Town (UCT) accepts the IB diploma as does the University of the Witswatersrand. When I wrote the National Education Department, they were still investigating the possiblity of drawing similarities and contrasts between the two systems.
An average offer for IB to UCT is 32-35 points including Maths HL, English HL, and a science.
I have a friend who is finishing IB at AISJ shortly, if you would like to get in touch with him, let me know.
Good Luck.
John