We did a trip for 20 days in July with Tracks4Africa paper maps and Tracks4Africa on an iPhone 4 with a local SIM card brought in the Windhoek airport from the MIC stand. The guy loaded the SIM in for me and added most of my credit as data. The Tracks4Africa app on the iPhone has all the map data already loaded so the there is no need for cell coverage or internet access.
We also loaded the Tracks4Africa maps on an iPad 3 / 3G but no SIM card and it works fine on there as well. While not strictly necessary we found having the maps on the iPhone was a great distraction on the long trips - nice to hear it tell you that the next turn is 100KM away! Even in the middle of nowhere like around Nambwa campsite heading towards Horseshoe on strictly 4WD tracks it worked great with no cell coverage.
The Sim and credit was N$332 and I loaded it as 1GB data and the rest maybe N$60 as phone credit. As the only time we had a 3G signal was in Rundu and the rest was Edge or no signal I only used a few hundred MB of data. We only phoned to make reservations so the phone credit was enough for us.The MIC card did not work in western Botswana where we were for 4 days.
Get the App it's fun to have along, plus it has all the Padkos information from the web site which was very helpful.