Also spotted on flickr this one, Clowns hate tangelos - which even the wikipedia knows is from the 36th episode of "The Grim Adventures of Billy and Mandy" called 'Attack of the Clowns / Complete and Utter Chaos'. Other billboards spotted have the lines
"I'm a hot toe picker." - "My boogers itch." - " I've got a meat beard" and "Shiny shiny. Pretty pretty."
So what is the deal and why are random lines from a cartoon showing up on billboards like that? Well, duh, it's advertising of course! Any real cartoon fan is also a Cartoon Network fan for their daily fix, and these are billboards for the hip kids in the know and all others who still have their childlike unbridled curiosity intact. Remember kids: "Clowns hate tangelos. Messes with their equilibrium."