Before iOS4.0 clicking the home button on iPhone exits the application, and Apple had in their guide that programmatically exiting the application was not accepted.
now everything changed in iOS4.0, clicking the home button puts your app in a suspended mode (multitasking).. and I think there should be a clear way for the user to exit the app, like an exit button.
is it now OK with apple? and how can it be done?
No still shouldn't do this.
You have handlers for the different stages, so this is how you should do it. There's no point in exiting manually. If you restart the app, ideally it would start where you left off, so this is either by resuming or by starting and loading the old state.
No reason for exit.
Edit
As this keeps popping up again: iOS Human Interface Guidelines says "Don't Quit Programmatically". And we have seen many reports of apps that had calls to exit() in them in the past.
Exiting instead of suspending by setting the appropriate key in the Info.plist file is perfectly fine, of course - but that's not a dedicated UI Button, just application-specific implementation of program exit by the home button.