i wonder what type of application that are allowed to be started in the background/foreground when the iPhone boot , and are they limited to iOS 4 ?
Simple answer: None are allowed to.
Only Apple apps are allowed at startup, all others must be manually started.
I think voip applications. That's what Apple documentation states anyway and yes, they are limited to iOS 4.0. You should declare your application as voip and make according changes in the code. See the link: Background Execution
Background execution isn't supported by the iOS. Apple takes many steps like this one to prevent the intrusion of their customers rights to privacy. If an app would be able to start on startup, then it would have no problem recording all activity such as GPS, saved files, and keystrokes. Apple doesnt feature, and probably will not feature multiple process handling (more then one third-party process running at once).
来源：https://stackoverflow.com/questions/4318019/what-type-of-application-that-allowed-to-auto-start-in-iphone