Do Parents Have the Right to Search Through Their Teens' Stuff?

I heard about two incidents this week in which parents were looking through their teens' computers.  In one incident, the mother had received a negative call from a school administrator, and although her child was not being called out for drugs, drinking, smoking, or sexual behavior, she felt it was appropriate to read through her child's private Facebook messages. 

In the second instance, the mother had gone through her child's entire Facebook account and deleted people she felt her child should not be associating with. She encourages all parents to have complete access to their teenagers' facebook accounts and to read through them periodically.

I do not have access to my childrens' facebook accounts, though I am friended with them.  I do not have their passwords.  I do not read through anything on their computers.  I don't listen in on phone conversations.  I don't go through their rooms.

I talked about this with my seventeen year old.  I told her that I would not hesitate to go through my teen's things if I believed she were on drugs, drinking, having unsafe sex, or doing anything illegal.  She wanted to know if I would tell her first and I said no.  She was upset by that. My attitude is that I think in those cases a parent has a right to look through the teen's things when the parent thinks the teen is doing something illegal or immoral, but never just because the parent is being nosy or just thinks as the parent, he or she has that right.

On the other hand, I am very uncomfortable with a parent going through the teens' things if said teen is basically a good kid -- good grades, good friends, clearly not doing anything wrong.  Maybe the teen is smart mouthed at times, or rude, or something like that, but to me, if the teen is basically a good kid, honest, trustworthy, etc., why search through his or her computer?  I think teens have a right to privacy and I think, even more strongly, that it is normal for teens not to tell their parents everything.  It is part of the teen experience to have private thoughts, feelings, and conversations as they separate from their parents.

I wonder where my readers fall -- is it ever okay for parents to go through his or her teens' stuff, or is it okay whenever the parent wants to? Should your teens tell you everything? Or somewhere in between?  Please share your thoughts!