If you turn on Multitasking Gestures (Settings -> General -> Multitasking Gestures -> ON) you can go "back" one app by four-finger swiping the current app off the screen to the left (i.e. swipe your fingers from right to left). iOS automatically puts the apps "beside" each other so the swipe works even if the apps were previously not launched consecutively. I do this when reading www.jimmyr.com to return to Safari if it launches the YouTube app. I also use this if I forget I'm in Airplane Mode and I get the 'Turn Off Airplane Mode or Use Wi-Fi to Access Data' error message that gives me the option to jump directly to 'Settings'; once I turn Wi-Fi back on I four-finger swipe to get back to the app. It's on iOS 5 on my iPad-1 but I think you might need a iPhone 4 or 4S to get Multitasking Gestures on an iPhone (I can't find how to turn it on in the settings for my iPhone 3GS). Edit: thanks to baddox for pointing out this is an iPad-only feature (confirmed here: http://www.apple.com/ios/features.html), or on iPhones that have been jailbroken.