I saw this article on Jalopnik today, and the first thing that popped into my mind was how did Google miss out on this idea?
It's the perfect combination of serving Ads, and geo-location. Somebody needs to make this a reality, and I'd prefer it to be Google. (I'm partially kidding)...
...Hey Google I heard you were looking for somebody to head up the new Google Billboards department, wink, wink.