What is the expectation of privacy in a committed relationship?

So, you know me. I'm watching Secret Life last week. And the topic de jour was privacy. Turns out teenagers love snooping through each others' phones. And it got me thinking. What is the expectation of privacy in a committed relationship? Do you need to snoop? Are you allowed to? Do they tell you everything anyway? Should they? 

I think it all comes down to trust. If you trust your partner to not do anything horrible, then you don't need to check their phone/emails/etc for proof of it. And if you don't trust this person, what are you doing in a relationship with them? 

Trust is a mutual quality in a relationship, and if your partner trusts you, wouldn't they tell you about a compromising text and the situations surrounding it? 

People really just need to talk to each other! But I'm not sure I'm making any sense, so anyway. What do you think? What is the expectation of privacy in a committed relationship? What is your expectation of privacy?