Nice discussion in this thread, a pleasure to read, although articulating all the points of view and confuting the others would require far more space than available. So, here my 2 ¢...
To answer the original question about the "declining Empire", I believe the right answer is "yes": the USA empire is in irreversible decline. The world's clock - LUCKILY - will not rewind its clock, and those countries who are on the way of their independence (EU included) will not get back to be the king's subjects. The main reason for the decline? Like for ancient Rome, an empire whose existence relies on never-ending growth is not sustainable.
I believe that the end of this domination built on military, political, media and economic violence, will however leave the space to something better, and that USA will not end their days in misery and despair, but in a prosperous and peaceful way.
Senator Obama said it really clear in one of the debates against Senator Clinton. He said something which sense was USA becoming "leaders" instead of "rulers". He spoke - for example - of the possibility to meet leaders of countries considered "bad ones" by USA public opinion. Cuba, Iran, Venezuela... he said something like "USA has to stop thinking that speaking with its president is a privilege. The USA president is a leader of a country within many others". To me it does not require rocket science to prove that talking to people you disagree from is the only way to influence their choices. (If this was not the case, a game like Diplomacy would not have any sense, BTW).
The biggest obstacle to this "USA republic" is however its people. Citizens of USA seem to have - in general - a very naïve and superficial understanding of the planet they live on... Most dangerously they totally lack the capacity of seeing "the largest perspective", to understand that on this planet we are a single, interconnected system.
The "war on terrorism" is probably the most evident of this lack of understanding, but the lack of action on climate change is probably the most dramatic.
So, down with the empire and long live the republic. But the republic, as any republic, needs informed citizens in order to be strong and prosperous. As long as the average US citizen will remain so deeply ignorant about the facts of this world, USA will keep on experiencing a steep decline in quality of life, material wealth, security, hope and happiness.
I believe - though - that USA has an excellent record when it comes to people's action and quick change (one example for all: the civil rights movements in the 50's and 60's). US citizens seem to truly feel "special" towards their country and to be ready to sacrifice some of their personal life in the name of the country... If this is the case, I am positive that under the leadership of a smart person, USA will be able to raise from the hole it dug for itself and shine again.