The world is divided between nations that have more emigration than immigration, and nations that have more immigration than emigration. Whether a country is a net importer or a net exporter (excuse the possible crassness associated with these terms) is decided by many things: the two most important being the country's demographics and wealth. Countries that are naturally losing population and that have much money are importers, while countries that are naturally gaining population and poor are natural exporters. A country's physical proximity to other countries and cultural norms associated with immigration and emigration have a lot to do with it as well. Currently, Western Europe, North America, East Asia and some of the wealthier Middle Eastern countries lead the world in net immigration, while Central and South America, The Philippines, parts of Africa and India lead the world in net emigration.
But even in a country that is gaining through immigration, there are many emigrants, and in a country who is losing through emigration, there are many immigrants. Some of this is situational: people relocating to a country they have ethnic ties to, specialists moving somewhere where their technical knowledge is needed, even people moving to a better climate. But some of this is structural: countries with high basic literacy and semi-skilled workers can send their workers to the world's richest and mostly developed countries, while some of the world's poorest countries send their workers to these middle income countries. This immigration treadmill is already in place, and as the world's demographics and economy shift, it may become an even more important phenomenon.
The countries that are big immigration destinations, such as The United States, Germany and Saudi Arabia, will continue to be big immigration destinations. But the countries that have been supplying them with immigrants, (Mexico, Turkey and Egypt, respectively) are nearing the end of their rapid population growth. It may be that it will be only the more highly skilled and educated workers from these countries who will be emigrating. It is also likely that as their labor markets tighten, with declining population growth, that they will be attractive sources of immigration from their poorer neighbors.
And this is where the concept of the immigration treadmill comes in: these middle-income countries, with the institutions and middle classes of the first world, but without the widespread prosperity of the first world, will be exporting workers to the world's richest countries, while importing workers from the world's poorest countries. Cambodia and Laos will send workers to Thailand and Vietnam, who will send their workers to South Korea and Japan. The richer nations of South America will send emigrants to North America, while accepting immigrants from the poorer nations of South America. North Africa and the Middle East will supply Europe with workers, while their places will be taken by workers from Sub-Saharan Africa. And while it isn't certain, it seems possible that the money flowing into these poorer countries from a tightening labor market will speed development, relieving population pressures, and causing a positive feedback loop.
The immigration treadmill is already going on, although it is hard to say what impact it has on the global labor market, since much emigration and immigration is illegal and undocumented. It will also, of course, not go without problems, probably causing social and ethnic tensions in places. However, my guess is that with the current demographic shifts going on in the world, the immigration treadmill will be an important part of the world's social and economic movements in the next few decades.
-- is a fascinating tool for studying the movement of people between nations.