In 50 years time (assuming that Pakistan and India don't destroy the planet first), the face of world football will be completely different. Some established countries will remain near the top: Brazil, Argentina, and the better European sides. Some others, like China, India, Japan, USA, Cameroon, Nigeria, and other African countries, will emerge as world powers.
The reason for this is simple: Population. China has 1.4bn inhabitants, and the national sport is football. India has 1bn inhabitants, and the third national sport is football (after Cricket and Hockey). Japan has a population of 130m, and invests huge resources into promoting football as the national sport instead of Baseball.
The USA has immense resources, not to mention a country full of well-nourished, sport-hungry fans. In the USA, "Soccer" is already the most widely played sport amongst kids - ahead of American Football, Ice Hockey, and Baseball. Once this generation of children reaches their 20s, the USA team will be strong enough to challenge most of the South American countries. Once these people pass football onto the next generation, we'll see the USA beating most teams in the world.
Everyone knows about Africa. Nigeria has a population of 120m people. The children don't attend school all day (normally a rotation of morning/afternoon). What do they do in their spare time. Football. No sport challenges football in Africa, except around the Cape. We already see the likes of Cameroon, Nigeria, Senegal, and Morocco take on established countries and win. An African team won't win the World Cup this year, or in 2006, but I'd put my house on seeing the continent producing world champions by 2050.
Why some countries in Europe will fall and others won't seems to lie in the foundations laid before. Football is king in Italy, Spain, and Portugal. This isn't always the case in northern Europe. Sure, it's the national sport of almost all of the nations of Europe, but in England, Rugby, Cricket, and Hockey dilute the stream of youngsters taking up the game. Therefore, the northern Europeans will most probably cease to be major powers, emerging only now and then to challenge for the World Cup, and Europe's banner will pass to the Southern Europeans. This, of course, all depends on the development of the European Union. It is quite probable that by 2050, the EU will be a federation of one single political entity, but, assuming that the current countries continue to play football as individual nations, Italy, Spain, and Portugal will eclipse the British, Germans, French, and Dutch.