Update, November 2012: see the schedule of the Confederations Cup.
The Confederations Cup takes place one year before the Cup, and is disputed by eight countries: the host country, the champion of the previous World Cup (Spain), and each of the winner of the six Continental Cups.
Ideally, all the twelve stadia should be ready for the Confederations Cup; by staging an actual match, the stadium would be tested as to its infrastructure (pitch, seats, illumination) and its functionality (ways of access, tickets control, parking area, etc).
However, six of the twelve stadiums are already excluded from the Confederations Cup.
Four stadiums are confirmed: the National Stadium, in Brasilia (will host the opening match, and only that one); Maracana, in Rio de Janeiro (will host the final match); Mineirao, in Belo Horizonte; and Castelao, in Fortaleza.
Recife and Salvador are running to get their stadiums ready; the Confederations Cup will see some great matches (maybe Brazil x Spain or Brazil x England – or whichever other country wins the Eurocup), and will draw the attention of the World.
Hosting the Confederations Cup will bring exposure, free marketing and a lot of business for the host cities.