OK, Terry, I'll open. And, since people that get paid to think about things like this, let me quote a few websites:

Is the U.S. an Empire? By Paul Schroeder:

American Empire is the current rage--whether hailed or denounced, accepted as inevitable or greeted as an historic opportunity. Common to the discourse is an assumption, shared also by friends and foes abroad, that America already enjoys a world-imperial position and is launched on an imperial course.

Schroeder goes on to define empire as "political control exercised by one organized political unit over another unit separate from and alien to it." Does the US meet this definition? Technically, perhaps, as we do control Puerto Rico, Guam, and a few other small non-incorporated territories.

But most Americans don't see the US as an empire or having imperialistic designs on other countries. ...whereas the British were generally quite open about the fact that they were running an empire, few American politicians today would use the "e" word as anything other than a term of abuse. As the military analyst Andrew Bacevich has noted, this goes for both Democrats and Republicans. Speaking in 1999, Sandy Berger, President Clinton's national security adviser, declared that the United States is the "first global power in history that is not an imperial power." Hegemony or Empire? by Niall Ferguson

In a classical sense, the US is not an empire. Discounting Puerto Rico, America does not exercise formally, directly control other countries. Splutter all you want about Iraq, our intent is to get out at some point, hopefully in the near future. Our policy there is very similar to the American occupation of Germany and Japan: help them rebuild and get back on their feet.

However - when we start discussing formal political control vs. informal (one could argue "real" political control) then lines get quite blurry. In American Empire? by Dominic Lieven we have a slightly different definition of "empire":

a polity ruling over huge territories and many people, in which rule is not based on explicit consent by the governed; it is a very great power that defines the international system and is linked to a great culture.

In that sense, the charge could definitely be made that the US is - intentionally or not - an empire:

It is in US foreign policy that American empire becomes most relevant. It is necessary to put US foreign policy in the context of globalization. The US does not want to control aliens the way traditional empires did. But if you take into account globalization, no one chose to live in the world we live in and there are no extraterrestrial living options.

So, it all boils down to a matter of definition. IMHO, the term "empire" has very specific - and non too flattering - conotations, and I don't view the US as being an empire. So, maybe we need a new term - "psuedo-empire", "soft empire", or some such.

There is one other consideration, and that is historically, empires expand and contract as political control over other states grows stronger or gets weaker. Unless we annex Canada, I don't see this happening any time soon.