An assessment of the heightened interest in advertising on cellphones, as summarized by N.Y. Times reporter Eric Pfanner. Mobile phones are conventionally referred to as “the third screen, behind television and the computer,” Pfanner writes. And yet Bob Greenberg, chief executive of R/GA, an agency based in New York that specializes in digital advertising, says he actually thinks about a cell phone screen “as the first one…it’s with me all the time.”