In a world as fast changing as ours it's impossible to predict anything more than a decade, and even then you should expect to be horribly wrong.
However, here's a few things people seem to be missing WRT China:
1. It's actually already slipping a little bit as the cheap labor center of the world. Places like the Philippines and other poor areas are starting to take industrial growth bit by bit from China. Presuming this trend continues, we probably won't see major effects for at least ten years. However, as China gets richer and more affluent, its attractiveness to labor intensive manufacturing firms will decrease and they will begin off-shoring to places with greater poverty.
2. As a caveat of number one, China will have to float the yuan eventually. When that happens, it loses its near monopoly on cheap manufactured goods exports to the US and, if NAFTA is still in effect, Mexico can expect a boom.
3. China has a lot of corruption problems you guys seem to be completely unaware of. They present a clean face to the world, but right now the Chinese bureaucracy is riddled with graft, bribery, and self-interested bureaucrats. It's gotten to the point where some, small sections of the ruling class are considering introducing democratic reforms on a lower, local level to combat the problem. Regardless of how they do it, China has to deal with this corruption problem before it can become a true superpower.
4. More than two thirds of China's population still lives in gripping poverty. Paradoxically, there's this part of China that is comparable with the Asian Tigers of the early 90's, transitioning towards first world, and then there's this part that is still just as third world as the worst parts of India or other places. The Chinese government is doing a lot to address this problem -- in ways, you could say the Chinese government barely cares for its urbanized, wage-earning laborers and is concentrating on the agrarian country-side -- but it's not solving it, merely addressing symptoms of the problem. It needs to develop its rural economy and it's failing to do this in a lot of ways.
5. Tied in with the above, there's a middle class of wage laborers of ever increasing wealth and affluence. The first slip-up or recession will break the 'contract' the goverment has with these people: We will deliver economic growth and you will stay quiet. It's doing a lot of preparing for this by creating the world's most pervasive surveillance and police state, but even if they can beat labor unrest, any rise in violence is going to lead to manufacturing firms high-tailing it somewhere more stable. China has so far escaped catching the Western Disease, but it can't escape forever, and it's up for a reckoning on this matter. As the global recession deepens over the next year or two, things might get very hairy in some parts of China's new cities.
6. It is a great importer of primary resources. This isn't as much a problem for places like the US or Europe because, as developed economies, primary resource extraction does not make up the majority of economic activity. However, China is an industrializing economy where secondary consumption of primary resources makes up most economic activity, which has certain delitorious effects on growth limits.
7. China's very stable currency is almost entirely dependent on the import of US treasury securities. If, at some point, an American government comes along that starts producing a balanced budget -- or at least a decreased deficit -- China's ability to manipulate the value of the yuan at will becomes suddenly much more sketchy.
China has a lot of the things it needs to be a superpower, but it also faces a lot of problems it will have to overcome in the future. It is not, for instance, analogous to the US in the late 1800's when it overcame the old European economies and became the world superpower (in potential if not actuality). It has more in common with the other Asian economic miracles that preceded it; that is, it's experiencing a vast growth in inputs on an input-output model of economic activity, but not vast increases in the efficiency of the input-output relationship (significant growth of which efficiency being what makes the US economy so competative and attractive to capital investment). So even if the Chinese economy someday surpassed the US economy in absolute size, the US would still retain a significant competative advantage.
And the Chinese central bank is too dependent on the yuan peg to do something as silly as let it become the world reserve currency, thereby losing the tight lid they've managed to keep on it so far.