The irony of course, of putting up more signs is that it is contradictory to what you are trying to accomplish. The idea behind this law is to keep people focused on the road, pay attention to their driving, ect... In short: reduce distractions. Since texting for all but the best means your hands and eyes are occupied with pushing buttons on the phone and looking at what you're typing, this means you've got nothing to watch the road or steer with, and if you try to combine the two you'll only accomplish doing both poorly.
As for signs, well yes, we could make a law that says "no texting while driving". And then we could put up signs everywhere so that while people are driving instead of paying attention to the road, they can look at all the pretty signs we've put up along them.
At what point do we realize that this really isn't helpful anymore?