A little random Googling has revealed that the entire text of Hardt and Negri's Empire is actually available in .PDF online. The tiny bit I was talking about last night is page 154-156. Here's the definition of Empire from the preface, for those who haven't read the book and want to get a flavor of what it's all about:

The concept of Empire is characterized fundamentally by a lack of boundaries: Empire's rule has no limits. First and foremost, then, the concept of Empire posits a regime that effectively encompasses the spatial totality, or really that rules over the entire "civilized" world. No territorial boundaries limit its reign. Second, the concept of Empire presents itself not as a historical regime originating in conquest, but rather as an order that effectively suspends history and thereby fixes the existing state of affairs for eternity. From the perspective of Empire, this is the way things will always be and the way they were always meant to be. In order words, Empire presents its rule not as a transitory moment in the movement of history, but as a regime with no temporal boundaries and in this sense outside of history or at the end of history. Third, the rule of Empire operates on all registers of the social order extending down to the depths of the social world. Empire not only manages a territory and a population but also creates the very world it inhabits. It not only regulates human interactions but also seeks directly to rule over human nature. The object of its rule is social life in its entirety, and thus Empire presents the paradigmatic form of biopower. Finally, although the practice of Empire is continually bathed in blood, the concept of Empire is always dedicated to peace—a perpetual and universal peace outside of history.

Hence the importance to Empire of the figure of the terrorist, who does not stand outside the Empire as an enemy nation might (because there is no outside in which to stand) but who is instead always the hidden enemy within.