This is just a hunch; not sure if this approach will work. If you look at phrases and the closeness of words, perhaps you can create a Markov chain? Thus, you can get an idea of the frequency of some phrases / words in relation to others (based on the order of your Markov chain).
So, you are building the Markov chain and frequency distribution for 2009. Then you create another one at the end of 2010 and compare the frequencies (of certain phrases and words). You may need to normalize the text.
In addition, something that comes to mind is methods of processing a natural language (there is a lot of literature related to the topic!).
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