I doubt this is possible with current technology - so many people share the same or similar writing style that, even with a skilled human analyzing, you probably can't do much better than attribute it to a large bucket of perhaps thousands or tens of thousands of individuals.
Even if you couldn't narrow it down to an exact person, it seems like you could get good accuracy on socioeconomic status, cultural background, political slant, etc. Once you had that data it might be valuable from a marketing standpoint. It might not be useful for sites where there is a very specific user group, but for sites with incidental user generated content--even if it isn't anonymous per se, you could get a more accurate picture of your audience.
