On some aircraft a transmitting phone signal from certain locations in the aircraft will absolutely cause an audible interference on the cockpit radios. One type of regional jet I've flown in my career would react to transmitting phones from rows 1-3. In that aircraft (ERJ145) I only observed communication radio interference, but there are reports from other types of aircraft that pilots have experienced interference on the navigation frequencies as well. The interference is spurious, hard to predict, and depends on specific aircraft model, specific airframe, and specific mobile device (Blackberries are particularly bad) ...which is why it's hard to conclusively prove. And sure, "No one's died yet!" because pilots are capable of dealing with communication interference without immediately crashing the airplane. That's why the FAA and airlines aren't rushing to ban all devices from the cabin, which is the other argument I hear a lot. ("If they REALLY caused a problem they wouldn't be allowed!") But still, if you know that sending your text is irritating the pilots with every click of the "send" button, isn't that reason enough to put it into airplane mode?