The sentence is saying that cell phone messaging is trendy, yes, but that's not exactly the way "sign of the times" is used here. "The times" means everything, the state of the world at present. The sentence is saying something like, "Just as you might expect, given the way technology is racing along these days and young people are all plugged into their individual devices 24/7 and so forth, the advertisers found that their most effective marketing tool was cell phone short messaging, as opposed to older forms such as billboards or ads in magazines, radio, or television."
And saying it in much fewer words!
I don't watch Nightline, but "Sign of the Times" without any preposition could be the title of a feature: here is an example of yet another sign of our times.
As vs. In: It seems to me that "in" was chosen here to indicate that cell phone messaging is in a group of many such observable signs of the times. "In a sign of the times" refers to the phenomenon of cell phones being effective in this way. You could say "As a sign of the times, we see companies using cell phone short messaging as an effective advertising tool," but that has a slightly different meaning: the usage of cell phones, and not their effectiveness, is "the sign of the times."
But I hope somebody else will weigh in on this because I feel I have become entangled. :)
[native speaker & writer, not a teacher]