There is such software. But Im not aware of any free ones.
Some crappy simple ones are scripts using wc, sort, uniq. They compare what words are used, how often, and the letter count on uncapitalized words. Not good enough for a conviction but good enough for a "who to watch" list.
Reminds me of one of the songs in the Dom3 Parodies thread joking about the possibility that some of the people in the games or posting here are actually devs or staff or infamous people from our past....
http://forum.shrapnelgames.com/showp...1&postcount=35