That's been around for a while - think of the video billboards of Times Square, or any of the ones that inhabit the financial districts of major (Western) citie

That's been around for a while - think of the video billboards of Times Square, or any of the ones that inhabit the financial districts of major (Western) cities. The problems come, I would guess, from a very high start-up cost. Not only do you need the side of a building in a busy location where people can stand, but you need a big-ass screen that is readable and reliable. You may also have trouble with determining advertising numbers - you don't even have the "subscriber base" numbers that print media has; all you've got is the average number of people that walk by, times some small fraction who, on average, watch the thing.