I just read on the website of LAM (Mozambican airline) that "passengers flying in the routes Johannesburg, Nairobi and Dar-es-Salaam that they must hand yellow fever vaccination certificate, otherwise passengers will be obliged to vaccinate at their own costs." (updated 03 July 2009).
I googled a bit, but did not find any hints that Mozambique has changed its normal Yellow Fever policy which is saying:
Yellow Fever Certificate
All travellers entering Mozambique who have been to any country where Yellow Fever is known to exist (as listed by the World Health Organisation) must present a valid Yellow Fever Vaccination Certificate on arrival. If you cannot present a valid certificate on entry you may be vaccinated at your expense (approximately US$55).
If you cannot present a valid certificate on entry you may be vaccinated at your expense (approximately US$55). src.: FCO UK Mozambique travel advice
Does anyone know what is going on? According to the WHO South Africa is a country without any known risk for Yellow Fever...