One concern is that iPhones and iPads carry an irresistible temptation to do something other than concentrate on the Bible. You can listen AND see what others are doing on Facebook or Twitter. You can listen AND shoot a text to a friend. Which of course, seriously reduces one's ability to actually listen and concentrate on God's Word.
However, my main concern is another subtle temptation: The temptation to turn the device off during the sermon. For example, I preach for about 45 minutes and I usually refer to particular words, phrases, and verses all the way through the sermon. Each and every point I make is hopefully tied to the passage I'm preaching from. I don't want people to assume I'm saying true things, I want them to see it in the text itself. I do not just read a text and then tell unrelated stories for 40 minutes (if this were the case, I'd encourage you to bring your iPhone and read a good expository sermon instead).
But, iPads and iPhones turn themselves off after a few moments to preserve battery life. Or, they are intentionally turned off by the user to preserve the quickly fading battery. What this means is that the Bible is not "open" during the whole sermon. The listener cannot just look down to an open Bible and see where the preacher is getting his point from. He's got to turn the device back on, be distracted by the picture on the welcome screen (tempting him to daydream about the beach from the family picture), and then "slide to unlock." By this time, you've probably missed the life-changing point the preacher just made about the word, "therefore."
Now, please don't hear what I'm not saying. I'm not saying you are more spiritual or a better listener if you bring your Classic Reference Bible to church. I'm positive you can listen well with a fully-charged electronic device (I hope so because I hear they are the wave of the future). I'm simply asking you to evaluate whether it helps you listen and follow along or whether it tempts you to "check out." If you are a person who does everything on your iPad all week, it may be a wise idea to bring your physical Bible to church as a reminder that what is about to happen is different than everything else in my life (you are going to hear God speak). It may help you engage with the text and listen attentively if you have nothing else to distract you.