That's a risk with any phone, and one of the reasons I don't bother with mobile-phone boarding passes. The other reason is that the few times I've used them, it's generally more of a hassle to get my phone out, find the relevant e-mail, click on it, and then (inevitably) have my phone screen go black just before I get to the agent so that I need to unlock it to show it to him/her. This takes two hands, so now I've got to put my stuff down, fumble with my ID...it's just a PITA. Paper works just fine and it's easier.