One of the up and coming new Facebook Applications is Honesty Box. When you install it, you can send an anonymous message to any of your facebook friends. Only that friend sees the message, along with whether the sender is male of female. The creators say that they will soon add the ability for people to respond to messages. In the user reviews, some people have asked for the ability to have messages appear to everyone on their profile. I suspect they’ll add that as an option in the future, too.
But for now, if you’re dying to tell someone that you have a crush on them, or perhaps complain about their body odor, Honesty Box may be just the application you were looking for.