No one here knows anything about what Apple might do in future versions of iOS. As of right now, except for those apps that can be turned off in the Restrictions, apps cannot be passcoded other than third-party apps whose developer has included such a feature, as is possible with some photo apps, for instance. The only other option is, as killhippie said, the screenlock passcode. You can comment to Apple on the matter, if you wish:
It may be a wise policy to just not keep potentially embarassing content on your iPhone or any other highly mobile device.