9. At a computer system including one or more processors and system memory, the computer system also including a plurality of documents and a document similarity index, the document similarity index indicating similarities between different documents in the plurality of documents, a method for updating the document similarity index, the method comprising: an act of accessing a batch of documents; for each document in the batch of documents, an act of computing a tag index for the document, the tag index including one more keyword/weight pairs, each keyword/weight pair mapping a keyword to a corresponding weight for keyword to indicate a significance of the keyword within the document; for each document in the batch of documents subsequent to computing the tag indices: an act of identifying a specified number of the most significant keywords in the document based on weights in the tag index; for each keyword in the specified number of most significant keywords, an act of determining the corresponding weight of the keyword in each document in the plurality of documents and in the batch of documents; an act of identifying a plurality of candidate documents, from the among the plurality of documents and the batch of documents, based on the corresponding weights of the specified number of the most significant keywords in the plurality of documents and in the batch of documents, at least some of the specified number of the most significant keywords in the document also being significant keywords in each of the plurality of candidate documents; for at least one candidate document identified from within the plurality of documents: an act of calculating a full similarity between the document and candidate document by determining the weight of additional keywords from the document within the candidate document; an act of identifying the weakest similarity, from among a specified number of top similarities, for the candidate document from within the document similarity index, the weakest similarity indicating the similarity between the candidate document and another document in the plurality of documents; an act of determining that the candidate document and the document are more similar than the candidate document and the other document by comparing the calculated full similarity to the identified weakest similarity; and an act of replacing the weakest similarly with information from the calculated full similarity within the document similarity index to incrementally update the document similarity index, the replacement based on the determination; and for any candidate documents identified from within the batch of documents: an act of calculating a full similarity between the document and candidate document by determining the weight of additional keywords from the document within the candidate document; an act of selecting a prescribed number of candidate documents for inclusion in the document similarity index as documents that are similar to the document, selection of the prescribed number of candidate documents based on the full similarity calculations and in accordance with one of a hard limit or an express threshold, the hard limit or the express threshold limiting the number of candidate documents that can be selected for inclusion in the document similarity index; and an act of storing information from the calculated full similarity between the document and the candidate document in the document similarly index.
9. At a computer system including one or more processors and system memory, the computer system also including a plurality of documents and a document similarity index, the document similarity index indicating similarities between different documents in the plurality of documents, a method for updating the document similarity index, the method comprising: an act of accessing a batch of documents; for each document in the batch of documents, an act of computing a tag index for the document, the tag index including one more keyword/weight pairs, each keyword/weight pair mapping a keyword to a corresponding weight for keyword to indicate a significance of the keyword within the document; for each document in the batch of documents subsequent to computing the tag indices: an act of identifying a specified number of the most significant keywords in the document based on weights in the tag index; for each keyword in the specified number of most significant keywords, an act of determining the corresponding weight of the keyword in each document in the plurality of documents and in the batch of documents; an act of identifying a plurality of candidate documents, from the among the plurality of documents and the batch of documents, based on the corresponding weights of the specified number of the most significant keywords in the plurality of documents and in the batch of documents, at least some of the specified number of the most significant keywords in the document also being significant keywords in each of the plurality of candidate documents; for at least one candidate document identified from within the plurality of documents: an act of calculating a full similarity between the document and candidate document by determining the weight of additional keywords from the document within the candidate document; an act of identifying the weakest similarity, from among a specified number of top similarities, for the candidate document from within the document similarity index, the weakest similarity indicating the similarity between the candidate document and another document in the plurality of documents; an act of determining that the candidate document and the document are more similar than the candidate document and the other document by comparing the calculated full similarity to the identified weakest similarity; and an act of replacing the weakest similarly with information from the calculated full similarity within the document similarity index to incrementally update the document similarity index, the replacement based on the determination; and for any candidate documents identified from within the batch of documents: an act of calculating a full similarity between the document and candidate document by determining the weight of additional keywords from the document within the candidate document; an act of selecting a prescribed number of candidate documents for inclusion in the document similarity index as documents that are similar to the document, selection of the prescribed number of candidate documents based on the full similarity calculations and in accordance with one of a hard limit or an express threshold, the hard limit or the express threshold limiting the number of candidate documents that can be selected for inclusion in the document similarity index; and an act of storing information from the calculated full similarity between the document and the candidate document in the document similarly index. 10. The method as recited in claim 9 , further comprising for at least one other candidate document identified from within the plurality of documents: an act of calculating a full similarity between the document and candidate document by determining the weight of additional keywords from the document within the candidate document; an act of identifying the weakest similarity, from among a specified number of top similarities, for the candidate document from within the document similarity index, the weakest similarity indicating the similarity between the candidate document and a second other document in the plurality of documents; an act of determining that the candidate document and the second other document are more similar than the candidate document and the document by comparing the calculated full similarity to the identified weakest similarity; and an act of retaining the weakest similarity with in the document similarity index based on the determination.