The question is whether there is an empire and if there is one, then what are its boundaries, conceptual or geographical? What is imperialism?
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We go, like a perpetually moving pendulum, back and forth on questions of whether or not there is an American empire and, if so, whether or not it's declining and, if so, against what measure (or, as Doug put it, "compared to whom") it's declining.
There are facts to cite and reasonably well thought out opinions to offer but at the end of the day it proves to be a slippery thing. I wonder if any Romans had an accurate sense of their polyglot state's position during that long moment when overwhelming power was still in evidence but trouble clearly could be seen on the horizon.
In any event, I think Leo Panitch has developed a good theory of contemporary imperialism which can be summarized as a system of interlocking dependencies among the so-called core states.
But there's no need to take my word for it. You can examine this yourself.
audio (interview with Doug) --
<http://shout.lbo-talk.org:8000/content/lbo/RadioArchive/2004/04_03_25_16.pls >
text (essay outlining Panitch's ideas on modern imperialism) --
<http://www.yorku.ca/socreg/Panitch and Gindin 04.html >
.d.