It was one afternoon when I was on my way to an exhibition at the Folger Shakespeare Library called “Breaking News: Renaissance Journalism and the Birth of the Newspaper” that I caught my first glimpse of the revolution in advertising. I saw it on the side of a Washington Metropolitan Transit Authority bus: a long, clean, pastel-blue rendering of the words “Yes You Can” with the o of the word you filled in by a version of the long-familiar, red-and-blue, yin-and-yang logo for Pepsi-Cola, now redesigned to look as much as possible like the hopeful-sunrise-on-a-ploughed-field Obama logo. There was no mention of Pepsi that I could see, but then I wasn’t looking closely at the words. I seem to remember something about deliciousness—was it?—or maybe youth and excitement in what amounted to a screen crawl at the bottom of the ad, but I didn’t bother trying to read wha ...