Just to update, we made it into South Africa without a carnet (never mentioned, asked for or anything). However MAKE SURE you get a temporary import form on the South African border (several people I know have had to insist to get one). It is impossible (ok, annoyingly time consuming) to ship your bike out without this.
Just to re-itterate Javier Carrion's line: A carnet is not needed in West Africa.
We were rarely asked for it at borders and not having one was only ever an issue entering Senegal. The countries we passed through were:
Morocco, Mauritania, Senegal, Mali, Burkina Faso, Benin, Nigeria, Cameroon, Gabon, Congo, DRC, Angola, Namibia and South Africa.
Vladivostok - Denmark 2008... Finished.
Denmark - South Africa 2010-2011... Finished.