12. A computer program product for use at a computer system, the computer program product for implementing a method for identifying key phrases within a document, the computer program product comprising one or more computer storage devices having stored thereon computer-executable instructions that, when executed at a processor, cause the computer system to perform the method, including the following: access a document; calculate 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, and output a list that contains at least some of the textual phrases together with the frequency of occurrence of output phrases within the document; access 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 output list, determine the expected frequency of each such textual phrase by using the expected frequencies of at least some of the individual words of the language model to interpolate from the expected frequencies of said at least some individual words the expected frequency of each such textual phrase contained in said list; for each textual phrase in the output list compute a cross-entropy value computed from the frequency of occurrence of the textual phrase within the document and the determined expected frequency of occurrence of the textual phrase within the language model; select a specified number of key textual phrases based on the computed cross-entropy values; and populate a key phrase data structure with data representative of each of the selected specified number of key textual phrases.
15. The computer program product as recited in claim 12 , further comprising computer-executable instructions that, when executed, cause the computer system to use a weighting function to weight the statistically significance of the textual phrases in said list relative to one another.