Graffiti Wall

Does anyone know of a plugin that will allow me to post some sort of graffiti wall? In other words, I’d like my users to be able to write short messages, almost like a bulletin board. Someone else would be able to write one directly below it.

I suppose the idea would be similar to a facebook wall.

If there is no plugin or script that can do such a thing, is there a way to somehow hack a comments field up to do it?