http://www.hackerfactor.com/GenderGuesser.html
See this paper:
http://u.cs.biu.ac.il/~koppel/papers/male-female-text-final....
Sidenote: this was an epic win for me in Gender Studies class, when I said that not only was the professor's claim that the genders communicate "essentially identically but with some individual variation" wrong, but I would constructively demonstrate it by blind-assigning a stack of any papers she cared to give me. (I also gave her a copy of the above paper, which she read with interest.) The class then retreated from the scary notion of binary decisions on the basis of the scientific method to the more comfortable one of endless arguing whether the differences were socially constructed or biological.