This showed up on Cheezburger.com, and was captioned to reflect the importance of commas. It's a very important lesson, and one that bears repeating here. Those of you thinking about having statements inscribed on your tailgates, please make sure your punctuation is correct.
There's probably 24 hour grammarians you can call, who will show up at your location in minutes, if there's some sort of grammar or punctuation-related emergency. In this picture, it's actually not entirely clear if this is an error— the truck owner may genuinely be calling for his fellow citizens to join him in riding some boys. However, If he's just imploring "boys" to just ride, with no further goal specified, then the owner should really invest in a comma.
"Let's ride boys" = "We should seek out boys to ride, whatever meaning of "ride" you'd like to choose."
"Let's ride, boys" = "All of you young males in the vicinity, let's proceed to travel, by vehicle."
Hope that helps. Now don't get me started on the semicolon issues I see airbrushed on van sides.
(Hat Tip to Luke!)