This ones been discussed here before once or twice. I thought of it a few months ago. I know in my city a large billboard costs several thousand dollars - my city isn't that large but it's not small either. I had considered them making my car note, insurance, plus gas with the guarantee from me I would put X,XXX of miles per month on my car - 2,000 miles was my magic number as I drive a lot. Somewhere along the lines of $500 per month until the fad wears out - the Smart being the latest fad.
The one consideration was the liability issue, if I do something wrong with that company name on the side of my car. I am going to do a good sized magnet up for the side and park it outside of my buddy's business for a couple hours. That'll be for free though.