I predict (and hope) that if interconnect rates do drop, Cell C will be the first to gain from this, and the first to drop its call rates (due to reduced costs overall), and will see increased subscriber numbers. The knock-on effect of this will be more users on its own network, meaning less off-net calls, which in turn means less interconnect charges, resulting in even further savings.
This will eventually cause MTN and Vodacom to open there eyes to this reality, and hopefully follow suit. However, Cell C will have to launch a 3G/HSDPA service, otherwise it will lose out on the higher end customers, who will not be willing to switch over due to lack of a data service (EDGE just doesn't cut it nowadays).
Meanwhile, Telkom will do well on the data front, with its 7.2mbps HSDPA with HSUPA offerings. Telkom already owns the underlying infrastructure to connect it's base stations, and will offer a superior data service on its very new GSM/UMTS network. Expect to see Telkom increasing its coverage very rapidly, with lots of base station sites acquired already (radio/microwave sites, Wimax sites, the 1000's of exchanges countrywide, etc). Telkom also already has a growing retail distribution network (Telkom Direct shops, possible partnerships with the Post Office negotiated with government).
Telkom has a lot going for it in order to launch a mobile service. The one thing standing in its way is the interconnect rates.