There are several key points to remember when planning for the weather in next years 2010 world cup. A lot of people hear that it will be winter and imagine one of two scenes, heavy rains or scorching sunny days (it is Africa after all).
Coastal cities such as Cape Town will be cool, around the 20 degree mark with a few nice days of mid 20's if we are lucky - with a dose of rain thrown in. Durban should be a few degrees warmer as they have warm air blowing in off of the Indian Ocean (Cape Town has the Atlantic ocean which is can produce much colder weather as well as storms and waves over 100ft - great for the surfers).
The interior of South Africa can be a mixed bag, it is possible to have hot weather that hits the mid 30's for days at a time (or even weeks), even if the temp isn't high there will be very little rain.
Having lived in Cape Town for three years June tends to be nice cool weather, the real heavy rainy seasons tend to kick in around August and the real hot sunny days are around Nov to April.
Where a team plays the majority of its games will have a big impact on the outcome of their tournament, certain teams in northern Europe might fair better in Cape Towns moderate climate where a southern European team might do better in Durban or a central city such as Bloemfontein.