The US will not be the world superpower forever, certainly, but those calling for imminent demise are being overzealous and premature. Lets look at some of the claims that apocalypsists make and reasons why the US isn't going to recede just yet.
1) The US owes huge amounts of money to other countries.
We do, but the majority of our debt is to individuals and private corporations, through bonds and the use of the dollar as an international standard. What would need to happen for this to become a problem? The world would have to lose faith in the security of US bonds(currently seen as the most secure investment possible) and/or the dollar would have to lose its place as international standard. Certainly, the whole implosion of Wall Street doesn't help confidence in the US government, but it is a worldwide crisis. The government isn't anywhere close to becoming so insolvent as to need to engage in inflationary money printing. In fact, many other nations currently have much higher debts as a proportion of their GDP than the US.
As for the dollar losing it's place, there simply are no better alternatives. The Euro suffers from many of the stifling economic policies of EU countries. The Yuan(China) is current set at a fixed price(too low) by the Chinese government to encourage exports and thus is useless as a standard. If China did let the market value to Yuan, it might eventually be able to catch on, but it would have severe economic consequences in China and require a fundamental realignment of their economy to be successful. Authoritarian regimes are also not popular for economic standards because the government has the power to potentially cause massive economic change very quickly with no resistance, as opposed to democracies, where debates and legislation slow things down.
2) The dollar is in a free fall.
The use of the dollar as an international standard is discussed, above, but here we examine the claim that the dollar is quickly spiraling towards worthlessness in the international world. Simply put, this is dependent on imports vs. exports. The US has been importing more than it exports, known as a trade deficit. If the dollar is strong, the trade deficit should go up, since imports become cheaper for the US and exports more expensive for everyone else. This demand causes the dollar to fall. Right now, with the dollar low, imports should become more expensive and US exports cheaper, causing the opposite effect. This is the position of the Milton Friedman school of thought concerning economics.
3) Who will fill the void?
Russia lacks the economy. The government gets its income from oil and natural gas, which is unsustainable. As energy costs rise, it forces research into efficiency and alternate energy. Additionally, this source of income is no more isolated from world economics than any other. The financial crises hurt a lot of industries and as a result, oil has been tanking. Russia and OPEC are starting to feel the pain. Other than its oil, Russia has no economy, and the government is known for destructive seizures of assets. Not exactly the best way to build markets.
China, the popular pick, certainly has a stronger economy than Russia and has seen monster growth in recent years, but they have a number of problems that they will have to reckon with as well in the coming years. Pollution and environmental destruction are both huge. The government may not care about the morality of these things, but as they get worse and worse, the economic costs start to pile up. Despoiled land leads to unsustainable resource use(for example, slash and burn vs. sustainable logging) and increasing health costs for the population. Additionally, China's non-market currency keeps the economy artificially shifted towards cheap exports. This is not the path to global superpower: the economy will need to advance grapple with the consequences of outsourcing. As China gets better technology and education, the value of labor will be driven up, and the cheap manufacturing sector will face competition from lower wage workers in poorer developing countries like Thailand. Finally, China is dependent on having wealthy consumeristic nations buy their goods. Take away the power of the US economy, and China loses one of their biggest customers. The financial crisis will have plenty of effect there as well.
I'm not saying China will never become a superpower. I think they probably will eventually, but I don't think that they are ready to step into that role in the immediate future.
The reason I believe China will become a world power is the fact that the country is taking steps to make itself a world power. Currently, The United States, Russia and China are the countries that can change the world in a single day. Also China is buying up American debt to keep us afloat (we buy their products). As of March of 2006, China held over $321 billion worth of U.S. Treasuries which are up from the $60 billion it owned at the end of 2000. In a single moment the leaders of China can snap their fingers and they can literally hold America by the balls. Sure, the Chinese economy will suffer a bit but, I'd sacrifice a little bit of my economy to remove a world superpower as competition.
How? They can sell the bonds to someone else, but bonds have a set rate of maturation. You can't just come demand all the money whenever you want, you get whatever maturation value your bond is at when you redeem it. China could stop buying bonds, which takes away a US financier, $300 billion isn't enough to cause the government to collapse. Hell, we just threw $700 billion at our economy. China would have to cause a crisis in confidence concerning US bonds to really damage us, and I don't know how they would do this(I also don't think they would engage in such economically self-destructive actions).