I try not to get involved with these things. I've only "slammed" a couple of people who went after me personally trying to make it look like I had no clue what I was talking about or that my oppinions weren't really valid (at least in comparison) and therefore wrong. If it doesn't concern me personally, I try not to get involved. Of course, I also can't claim ammunity as I haven't always lived up to this standard here myself.
I must admit though that I find this "slam book" idea interesting. I never saw one when I was in Jr. High. Of course, in my Jr. High (or "Middle School" as we refer to it in Seattle) students weren't afraid to just start rumors or write their insults on the walls. I'm not sure that this method would be any better than the book.