I'm not a programmer, so please correct me if I'm wrong.

The basic idea for such an application would be a frontend like applocker which maintains a list of all the applications on the phone and the basic algorithm for the daemon would be..

If selected application = protected application

trigger password prompt

else, launch application

if entered password is correct, launch the application
else, display "Incorrect password" and terminate the appication before launch.

I'm not a programmer, this may not be this easy or my idea may be incorrect. Please do correct me.