LOS ANGELES — Senator Barack Obama used one to announce to the world his choice of a running mate. Thousands of Americans have used them to vote for their favorite “American Idol” contestants. Many teenagers prefer them to actually talking. Almost overnight, text messages have become the preferred form of communication for millions. But even as industry calculations show that Americans are now using mobile phones to send or receive more text messages than phone calls, those messages are coming under increasing fire because of the danger they can pose by distracting users. Though there are no official casualty statistics,...