Locking functionality within iPhone/iPad

Yes there is a number lock to access the content of your iPhone/iPad. But why isn't there the option of locking access to emails or photos? You have a friend that wants to use your iPad but maybe you have some 'personal' stuff on there. So you let him borrow it for a second and all the while you are standing over his shoulder making sure he doesn't access your photos of your .... *ahem* well we will just leave that to your imagination. It would be nice to 'lock' apps within iOS so that prying eyes aren't looking at things they shouldn't.