You can't, because Safari is now open and your app is now closed. In addition, the iPhone is sandboxed so you are probably not allowed to access Safari's information anyway.
Instead, I would recommend instead using a UIWebview. It allows the user to access the web from inside of your app. The current URL open in the UIWebView is located at myWebView.request.URL.absoluteString;
EDIT: This is not an example of multitasking. Your program can not run in the background unless it is making a VoIP call, checking location, or receiving push notifications.