PRIVACY - PIN PROTECTION FOR SELECTED APPS

While I am happy to let kids play Monkeyball on my iPhone, I don't want them to EVER open my Mail app, and while I am happy letting curious friends try out my iPhone, I don't want them checking my Calendar. The ability to lock certain apps with a four digit pin code would be nice. Should be applicable to ALL apps, including Settings.

I imagine the process of locking apps would be done by simply adding the desired apps to Privacy's list via Settings.

Alternatively, Privacy could work like a pin code locked gLauncher (on Cydia now).