You can always add a password to the lock screen if that is a concern. I know I'm in the habit of locking my iPods screen basically any time I set it down. It won't stop the kids from getting at the mail client if you hand over so they can play games or watch a movie... which gets back to my wish of better parental App control.
Even without multi-users it would be nice to lock out single apps and allow for a password override to launch.