There is no way to do this.
iOS 3 (which you've tagged your post with), there's no support for multitasking; your app doesn't run until it's reopened by the user.
In iOS 4, most apps will be moved to the background, but they're only permitted to do anything in the background for a few limited purposes—completing outstanding tasks that were running, playing music, listening for incoming VoIP calls—and the user will have to reopen the app either in the usual way or by opening the app tray (by double-tapping the home button) and selecting your app.
The primary advantage of the iOS 4 approach for you as an app writer is that you don't have to save or reload user state in your app; the user will return to exactly where they were before the interruption.