I'm not sure if it's still happening. But I remember hearing a couple of years ago that an advertiser would help pay for a new car if you let it be a mobile advertisement. It ends up looking like the public buses that are completely covered (windows and all) with a form fitting advertisement. From what I understand you plastered advertisement is dependent on where/when you drive so that it's shown to the appropriate audience.
I saw a Cadillac Escalade covered like a Gucci bag... made me think twice if it was an ad or the person just loved Gucci.
There's a Gucci Escalade here also. White with multi-color dbl G's all over it like wallpaper. From the spinners and the whompin' bass, I don't think it was an advertisement. As pimped as that ride is, it has to be a Gucci lover.