Talking 'bout street sign graffiti, Karen of Say No to Crack put together a short list of some really funny ones.LinkBut really, don't do 'em - graffiti is graffiti is graffiti. Maybe except this one: Soap Not Spray Cans: Reverse Graffiti Art.
There's a funny Stop sign near my house that says "Hammertime" under it. It always makes me smile and remember when "U Can't Touch This" by MC Hammer was a huge hit. Apparently there are quite a few Stop signs with graffiti pertaining to song lyrics.