I would just like to clarify, regarding my own children. They were with dad for five years, they were 7, 9 and 10 when they went. The practice of opening and keeping their mail from family went on the entire time. It wasn't so much the opening as the holding gifts and letters hostage or throwing them away. And telling the kids they did so. It was a power play as were many other things.
They are now soon to be 14, 16 and 17. They bring the mail in every day and it comes straight to me. They wait to see if they have any mail after handing it to me. I give them their mail from family and friends, they usually sit down in my office and open it or talk about what their friends/family are doing and what they wrote about. They don't have to, they just do. Bills or stuff from school, I open that and will continue to do so until they graduate or pay their own bills. With the exception of letters regarding higher education, I am only concerned with "at the moment" communication.
I too have what some have called an unusual relationship with my children. I have to, they have serious trust issues with adults. We are very open about everything in this house, and they can and have come to me about things most would be surprised a teen would discuss with their parents. Even their friends, especially the boys, get a laugh at us. My boys are very open and tell me they love me in front of their friends, they hug and kiss me before they leave the house and do so when they return. They call me before they go to sleep at friends houses just to tell me they are ok, and good night I love ya mom.
I don't read diaries either. Unless* they were to give me reason to be concerned. Now I do monitor *everything* they do online. I have passwords to all accounts, and the master email for resetting passwords is my email. Computers and laptops are used in common areas of the house and the time is limited. I randomly go through their friends lists on FB and delete anyone I do not know or can verify they know personally. They do not have instant messenger access other then on FB; myspace is not allowed neither are webcams. The entire recent "jessi slaughter" incident online is a prime example of poor parenting when it comes to the internet and monitoring access.