1. At a computer system including one or more processors and system memory, a method for identifying key phrases within a document, the method comprising: an act of accessing a document; an act of calculating the frequency of occurrence of a plurality of different textual phrases within the document, each textual phrase including one or more individual words of a specified language; an act of accessing a language model for the specified language, the language model defining expected frequencies of occurrence at least for individual words of the specified language; for each textual phrase in the plurality of different textual phrases, an act of computing a cross-entropy value for the textual phrase, the cross-entropy value computed from the frequency of occurrence of the textual phrase within the document and the frequency of occurrence of the textual phrase within the specified language; an act of selecting a specified number of statistically significant textual phrases from within the document based on the computed cross-entropy values; and an act of populating a key phrase data structure with data representative of each of the selected specified number of statistically significant textual phrases.