They could be checking their email, doing a bit of surfing or checking some facts as they come up. A part from being occasionally irritating, is this doing any real harm?
Some research by Helene Hembrooke and Geri Gay suggests that it may. They gave college students laptops and asked them to attend a lecture. They asked half the students to use their laptops during the lecture (not unlike in a meeting) and the other half to keep their laptops closed (don't see that in meetings very often). The lecture was followed up by a test based on the lecture.
The participants that browsed with their laptops did considerably worse than those that left their laptops closed. Hembrooke and Gay also analysed the results and browsing style using Lang’s Limited Process Capacity model. The moral of the story is that browsing in a lecture is not a good idea. Your ability to take in information is impaired.
It is not unreasonable that the same would hold true in meetings. We have a limited cognitive capacity for processing information and if we are browsing something that interests us more than what we are hearing or seeing in the meeting, then we will allocate more cognitive resources to our browsing and take in less of what is happening in the meeting.
With smart phones, tablets, laptops all cable of allowing our scarce cognitive resources to wander you might consider asking attendees to turn off their devices . You have probably even seen this at home where your partner is having a conversation with you while browsing on their iphone. After a few minutes you figure out they have probably only heard half of what you've said.
So if the meeting you run needs full input from all attendees, if you need them devoting all their cognitive resources to the matters in hand, then a device ban may be in order. I sincerely hope you are not in a meeting reading this. If you are then make sure to read the minutes.