Buying a South African SIM on arrival is fine and simple BUT the South African government now requires registration, which can be time consuming and may be impractical on arrival. When I did it earlier this year, the retailer (Vodacom shop) required a written confirmation from my hotel that I was 'resident' there.

A more practical option could be one of the E-Kit SIMs. They are normally cheaper on e-Bay. This one may be interesting: the price of $9 includes $10 credit; calls are free to receive in SA; outgoing calls to US cost $0.49 per minute. These prices are for a +44 number, a US number is available.
http://cgi.ebay.com/ekit-USA-SIM-car...item20add40c84

I regularly use my E-Kit SIM in different countries. It works on the callback system - dial the number, wait for the system to call you back, answer and speak.

Other E-Kit SIMs are available: http://shop.ebay.com/?_from=R40&_trk...All-Categories