This probably won't be a problem for many people, but it's something I didn't think of and falls into the large category of "technological conveniences that can totally bork privacy." I have (or possibly had) a close friend who married a controlling psycho, and I knew he was screening her calls and texts when he could, but he was still getting way more information than either of us could figure out a reason for. Finally it dawned on me. He updated her iPad and therefore had access to ALL of her texts (and e-mails...change your password, honey), at least from people with iOS 5, which I think most of her friends do now. He sits there all day reading her texts. If he sees one he doesn't want her to, he evidently fetches her phone and just deletes it. It's creepy.
I know I should have thought of this before, but I'm not used to any of her gadgets being updated if I'm not the one doing it, and also I don't immediately suspect people of being that underhanded. I turned off iMessages on my phone for now. I don't send any top-secret texts or anything, but it occurs to me that I want them going only to the phone I send them to. It's convenient to get them on MY iPad, but I don't want mine going to the recipients' multiple devices, because, duh, I don't know who's got them. My friend has no idea how to set up anything (or turn it off, sadly) on her phone or iPad. I am responsible for setting up push e-mails to both. I didn't tell her how to protect her privacy because I assumed she'd be the only one looking at her own stuff, and I also would have assumed, had I thought about it, that she wouldn't give out her unlock code (but I guess you should be able to trust your spouse with your phone, right?). Dumb, dumb, dumb. And unfortunately her new husband DOES seem to know how to do all of this.
Highly unfortunate situation. I can't imagine living with someone who would secretly read all my texts for a month and randomly delete the ones he didn't want me to see. I really can't. I'm irritated it took me this long to put the pieces together. I'm usually a better geek than this.