My Idea: Create an auction based advertising system, similar to adwords, but where you sell advertising covering a car, rather than an online space. The drivers of the cars would be regular commuters who live, commute through, or work (and park) in a high-profile area. Drivers would apply by entering their home and work addresses, their average commuting route and time, and whether the car will be parked on the street or in some other visible location.
Upon successful match-up with an advertiser, each driver would be issued - for free - a brand new, low-cost, but noticeable car (like a Mini, a PT Cruiser, a Beetle, or a Zap/Smart Car). Each vehicle would be outfitted with a GPS tracking sensor so that the advertiser would have complete access to information regarding where and when their ad is displayed. (Other minutia like keeping the car clean and in good repair would be stipulated in the contract.)
Their Idea: http://www.getanadcar.com/
Basically exactly the same idea. Groan.