I think there's a difference between snooping because your teen isn't telling you everything, and snooping because there's some reason to believe she is involved in something dangerous.
Most kids reach a point (sometimes as young as 6) when they want to keep some things to themselves. Usually this is harmless. I feel it's important to respect that desire for privacy. If it's bothering you, I feel it's better to tell the child that and ask some questions about whatever it is you would like to know, than to snoop behind child's back while allowing her to believe you are respecting her privacy. If you assume that because you don't know everything about your child, she must be hiding some wrongdoing, that is a very negative view of her.
Even when things look suspicious, if danger is not certain and immediate, I think other strategies should be used before snooping. For example, in the other thread (in Talk Amongst Ourselves) someone mentioned a 16yo sneaking out at night and taking the car and going out to do unknown things. My first approach to that situation would be to say, "Look, I know you are sneaking out at night. That worries me because I don't know what's happened to you when I thought you were home and I find out you're not. I don't go out without telling someone in the family, and I expect you to do the same. I'm also wondering why you want to go out so late at night. Where have you been going?" Then I would listen to what she had to say and try to negotiate a mutually acceptable solution.
If that didn't work--if she wouldn't talk about it or flatly refused to keep me any better informed of her whereabouts--then I would take her car keys away. If she can't use the car responsibly, she can't use it at all. That wouldn't necessarily stop the sneaking out (depends on where she's going) but it might, and it would serve as a natural consequence.
If the problem continued, I'd talk to her friends and/or their parents to try to find out where she's going. Then I would tell her what information I had collected and how, and the next step would depend on where she's going--I mean, my reaction would be very different if she's going to buy heroin than if she's studying owls.
If I couldn't get any info from friends, I'd drink a bunch of coffee and stay up all night and follow her if she didn't see me, or intercept her. Following would be more informative, but intercepting might startle her enough to change her attitude.
If that wasn't successful, I would stop and re-think whether to be concerned about her safety. Okay, she is sneaking out, and I'm not going to stop demanding that she show the same courtesy as the rest of the family and let us know when she is and isn't home...but is there any other sign of a problem? Is she acting weird or hostile other than when I confront her about sneaking out? Is she still doing well in school, keeping up w/activities, maintaining her responsibilities in the household? If she seems okay in every way other than sneaking out, I would assume that whatever she's up to is not harming her. I would continue to be alert for signs of trouble and continue to demand that she tell us when she is going out and when she expects to be back, but I would try to relax about where she is going.
It's only after all the above had failed AND there was a clear reason to be concerned about her safety that I would search her room, listen in on her phone calls, or read her e-mail. Why? Because all the above steps are related directly to the problem, whereas snooping may turn up private information that's unrelated and none of my business, such as her efforts at writing erotic fiction. Because I wouldn't be able to use the info gained from snooping unless I told her I'd snooped, in which case she'd likely be so furious w/me that we wouldn't be able to have a productive discussion. Because snooping violates her trust in me.
As a matter of fact, I did some sneaking out when I was 16. My parents found out about it and got as far as the first approach above. I explained that when I couldn't sleep, I enjoyed going out for a walk because there was no traffic and everything felt so peaceful. We went over safety practices (stay away from dark or overgrown areas where I could be ambushed, carry ID, etc.) and agreed that I would do this only on weeknights after 2am (when they felt I was less likely to be hit by a drunk driver) and that I would always leave a note in a particular location saying what time I went out and what time I would return. I really appreciated being allowed this freedom, and I always followed the rules. It's true that ONE of about 30 such excursions was coordinated w/my boyfriend and that we had sex (using 3 forms of protection) but otherwise I really was just out for a walk and not getting into anything improper. I think my parents' approach was much better than snooping thru my room, which wouldn't have turned up any info about what I was doing when I sneaked out but would've turned up many jotted-down private thoughts on various subjects that I would've been embarrassed to know they'd read out of context, and if I knew they'd snooped it would cause a permanent rift in our relationship.
But even that (though would be hard for parents to stand) was great for me as 1) it was so very, very fun and 2) got it out of my system and I ended up marrying a very "good" boy.
I want to grow up to be Loretta--she actually has a teenager and is behaving the way I hope I will when I have one. ITA with Loretta's reasoning about the letters--would've been better not to read them, but it was hard to avoid when they were thrust into her hands, so apologize to DD before addressing the subject matter.
If only they'd started those discussions with, "You know that we don't snoop in your things; however, your aunt has different beliefs and read a letter you wrote to your cousin and called to tell us about it, and now that we know, we need some explanation from you," we all would have been a lot better off. 
Loretta!
