Rob Farley is decidedly unimpressed with Niall Ferguson's Colossus.
Personally, I didn't hate it as much (although I'm also not convinced). People with actual expertise tend to hate "bigthink" book by people outside their area. But it is a necessary genre, in light of how specialized academic work necessarily has become. Those of us who pontificate on the Internet can hardly get too snooty about fact-checked books.
Still, "bigthink" requires some level of logic, and Farley points out that Ferguson's central thesis -- America is an Empire, and it's a good thing too -- depends fairly crucially on what you mean by "empire." If it just means economic and political influence, then the thesis becomes a "provocative" way of saying America is a really important country. Farley also makes a pretty good point when he suggests Ferguson has an oddly Leninist idea of imperialism, even though he presumably doesn't buy the whole concept of surplus value which makes Lenin/Hobson's theory work. (That theory, in turn, can't survive the elementary point that a consensual transaction generates wealth for both parties to it.)
To some extent, we can just stipulate our definitions: as long as we don't define Rome and Britannia out of the Empire business, we can choose concepts that may keep Athens and the US in or out.
One thing I like about Ferguson, in comparison with the standard American commentator indignantly denying that his country has an empire, is that he knows enough about the British empire to realize that it (and for that matter the Romans) did not eliminate or try to eliminate all self-government for its colonies. He doesn't have a straw Empire in mind. On the other hand, he still needs to do some work to avoid equating influence and imperialism.
To me, the most straightforward definition of Empire centres around sovereignty. You could start with an idealized Weberian sovereign state -- it possesses a monopoly of coercive authority within a defined boundary. Federalism splits that idealized sovereignty in a symmetrical way. Empire, on the other hand, leaves the homeland fully sovereign, but also exercising some jurisdictions in the colonies (without necessarily exercising all of them).
There are types of coercive force that are reserved, in the contemporary world, to the US. There are also forms of sovereignty that only the US can really claim. Much of the liberal Euro-Canadian agenda is to transform these powers into something a bit more federal (at least within the "West") -- much of the Bush agenda is to resist this transformation.