While it is appropriate to turn off our phone during an academic presentation, it is impractical to demand people also shut down their laptop and tablets at conferences. Many of us use these to take notes, as we attempt to live paperless lives or we simply can type faster than we can write.
However, you must realize that how you use your laptop or tablet is crucial, for not only you and what you get out of the presentation, but also for how people perceive you.