I remember reading similar concerns when PBS came out with a radical new TV program called "Sesame Street." ;^)
It is much easier asking these questions than doing actual research and coming up with some answers. I think a lot depends on what they are doing with technology: if they are reading and learning or just goofing around and wasting time.
I taught college classes for a number of years. Eventually, it became very common for students to bring their laptops with them to class. Some of them followed my lecture notes and tried sample problems. Others read email, web sites, or played games with the sound turned off. As long as they weren't disruptive, I didn't try to stop them.
Of course, K-12 is very different than college, but when I was in high school, I carried a book with me to read when a class got boring. These days I carry several books on my phone in case I get some extra time. My grades were pretty good, so I didn't seem to suffer from not paying attention.
Essentially, the question seems to be: "Does the teacher have to keep the students entertained?" Perhaps it should be phrased: "Does the teacher have to keep the students involved?" Teachers that drone on endlessly, sometimes reading their lecture notes, will have problems. Those that interact with their students and have activities that involve the students will do much better. As always, anything that changes the current situation is suspect.