From a teaching note at my campus' faculty center:
Don't believe it when students argue that they can pay attention to what's happening in class and supplement the class material as they surf through the internet. Research is abundantly clear that people cannot pay productive attention multiple cognitive inputs at one time. “Heavy” media multitaskers, or people who frequently pay attention to multiple media inputs at once, are particularly bad both at paying attention to the multiple inputs and recognizing that they are not paying attention.
Some students argue that having other stimuli, such as music or tv in the background, helps them to concentrate on a task at hand. This may be true, but in a limited way: If the stimulus does not compete cognitively with the task at hand, it may be helpful in improving mood and time on task. So if students want to listen to music while doing their calculus homework, and it works for them, great – but reading texts while they're supposed to be reading and writing about what is going on in class, not great at all.
The ultimate result of paying poor attention is poor memory, which equates to poor learning. If a person does not pay attention to something, there is no way he can learn it.
So how do we sell multitasking students on the idea that they're not just distracting others and annoying their instructors — they're actually shortchanging themselves?