Plate reads: "Wife and Dog Missing. Reward for Dog."
Maybe in the best of all possible worlds, his wife has a similar plate on her vehicle, and this is some kind of inside joke between the two of them about how much they love their dog.
Or is this part of the terrible bargain his wife has regretfully struck to stay in the relationship, laughing at his oh-so-hilarious-because-I’m-only-kidding-but-it’s-funny-because-it’s-true little joke.
Also: Somebody thought this was funny enough to enough people that it would be worthwhile to design, manufacture and sell it.
Edited to add: Someone was looking at me funny while I was photographing the front license plate, so I was too shy to go around back and see if he had any of these.
Intransigentia's Blog 
Doooood! You beat me to it. I saw that picture, and before I even read below the cut, I thought: “I wonder if he has those stupid truck nuts on the back?”