I found this site today, so it will doubly serve as the public restroom graffiti post as well as the site of the day recommendation: The Writings on the Stall. It is a user-created collection of things that people have written inside bathroom stalls. Warning: some of these are rather vulgar.
"This is a tee pee
for a pee pee.
Not a wig wam
to beat your tom-tom."
And then there's this essay on the subject by Lightsey Darst.