I want to develop a screensaver-like app where the only way of getting back to the springboard is entering a code. Therefore I would need the Home Button action to be ignored, that is that when the user presses the button nothing happens..or something custom...instead of going back to the springboard and terminating the app. Is this possible? does anybody know if apple will approve/reject this kind of app?
Not with public APIS (and hopefully, not with private ones). Even if you manage it somehow, Apple will reject it for breaking UI guidelines. Moreover, even if they didn't, people could always use the iPhone's "Force Quit" equivalent to hard terminate your app.