The only reason they should have such devices out during the homily is to take notes and read the relevant Scripture passages that the Priest may be quoting (my wife does this), or perhaps go over the reading again. Of course, the kids (if approached) may ask why you were paying attention to them and looking at the iPhone screens and not listening to the Priest. The touchscreens are designed as such they make no sound, so all you’ll really see is just some finger movements.
The temptation is going to be there anyway even if you take away handheld devices. As someone else said, they’ll distract themselves with songbooks and conventional missals, using them in a distracted manner. As someone with ADHD, I would know

Are we going to remove those books as well?
Meanwhile a nook, kindle, and other e-readers can be very limited in their scope. If they do not connect to something like a 3G network there will be no Twitter/Texting/Facebook. There is a version of the iPad like this, and the conventional iPod Touch is like this as well. So unless the Church has an unsecured wireless network available (very, very very dumb if they did) those people wouldn’t be able to surf on the Internet.
As someone who typically gets Nintendo devices (such as the DS), the only way this is really applicable to me is playing audio files using headphones (at reasonable volumes of course) while at the Adoration Chapel, which I’m assuming due to the total lack of responses on that thus far everyone is fine with.
Of course, considering I placed my credentials in my signature here it should come as no surprise I’m pro-technology.