I agree with Will that the US was (probably) the first country to have the powers of an empire without actually having a physical empire. But that's missing the point of the question. Are these powers in decline?
Yes, certainly. I don't know how anyone could argue otherwise. Some things to consider:
1. The ability of the US to force its foreign policy on other people is limited by the power of other parties to resist. China is starting to resist. Russia is definitely resisting. India and Brazil are a bit behind, but they too are starting. The European Union is finding a new path which is arguably less aligned with the US than it was in the Cold War.
2. The US economy is in shambles. Never mind the optics of people buying PlayStations and flying to Hawaii for vacation. If it were any other country, it would be in an all-out depression right now. It's only hanging on because it is considered "too big to fail" by the rest of the world. Many commodities, including oil, are bought in US currency, so the health of that currency matters to people. And there are trillions of foreign dollars invested in US companies and resources, so again there are big interests in keeping things working. But there's a limit to how much the outside world can stomach. If the Fed keeps devaluing the US dollar by printing more and more money, those countries already threatening to switch to euros for purchasing oil will probably make good on their threats. And those nice Chinese investors (not the government, but the private middle-class individuals who invest in the US in these funny stock market/gambling houses they have in China) will stop investing and start asking for repayments of the trillion dollars they are owed. What then?
Yes, I would argue that in every conceivable way, the US is in decline. It is unavoidable, given the growth of other countries. The question will be whether Americans take this in stride and fix their economy - while also accepting that other countries will be powerful - or whether they turn to political extremism and blame the world for their ills.
And Will, spare me the arguments about American "exceptionalism". If I have a kidney stone, I'll pray to be a Canadian citizen. If I have to take someone to court, I'll pray to be Swedish. You get the point - the US system is not as grand as it is made out to be.