We are traveling to South Afica and after much research have decided to order a sim card online beforehand from B4i.travel. You can take your unlocked cell phones and when you arrive at the airport you can visit the store in the terminal where they install the sim card and get you up and running. You receive your local number before leaving. I had a ton of questions before ordering and the Nationals Rental Manager has answered every one of them in a detailed and prompt fashion over email.
I like the convenience of not having to deal with RICA registration. I thought the prices much more reasonable that when we took our local phones to Europe through the AT&T network.
Let me know if anyone else has had a good experience with this company.