Coke has a billboard covered in some kind of plants that it claims absorbs 46,800 pounds of CO2 a year. Forgetting for a moment whether we should care about CO2 abatement, I have a question for Coke: What are you going to do with the plants after you take down the billboard and/or after they die? Are you going to shrink-wrap them and bury them deep? Because otherwise, when they die, they are just going to give up the carbon back to the atmosphere as they decompose. If you really want to abate carbon with plant growth, go build some wood-frame houses.
Update: Incredibly, as the 13th commenter on the linked post, I was the very first who did not think this was an awesome step for the environment.