Does NOT work on iOS 4.0 or higher
iAccounts brings users to the iPhone and iTouch. This will allow you to lock certain apps, as well as lock other things such as: icon moving, searching, and even wifi locking! These options can all be set for custom users which you can configure to your own needs. You can then log in and out as many times as you want with any user you want! There is also a screen saver that runs when the iPhone or iTouch is dimmed, right before it sleeps
See More Info below for screenshots.