The answer is that there is nothing that allows a particular built-in area (texts, emails, address book, etc) of your non-jailbroken phone to be separately pass coded. The only option I am aware of is to have a pass code on the entire phone.
And I can see a number of situations where this would be helpful. For example, there are times when my kids want to use my phone because theirs isn't charged, etc. Sure, I can say NO. But I'd rather just feel like I can hand them the phone and not worry about the GF sending me a personal text. It's just that simple.
It's a feature request that I hope eventually makes it into IOS.