Haven't really seen any funny license plates, but a few funny bumper stickers. Just yesterday I saw one on a Toyota pick-up Truck that said "Keep tailgating me and I'll flick a booger at your windshield". Me, my dad and my sister laughed for a while about that one.
My Aunt Rose had one on her 1995 Chevy Tahoe 2 door (which she just got rid of because it was falling apart. She replaced it with a 1990 Chevy Celebrity station Wagon) that said "I'm not tailgating, I'm drafting", a referance to her being a big NASCAR fan.