There's really nothing stopping this happening from a technical standpoint. Have multiple "swipe to unlock" arrows on the lock screen, one for each user, labelled of course. Passwords on accounts if users so choose. Everyone gets their own apps, browser history/cookies/cache, music, movies, etc.
Where this gets tricky is when users of the same device want to share things. You shouldn't need to have two copies of the same movie on a device. I also have no idea how iTunes syncing would work.