How to Quit a Multitasking App in iOS 4
Multitasking has come to the iPhone, well it was there all along, but only for Apple apps, now with the release of iOS 4 any app can be developed to take advantage of it (if your iPhone supports multitasking).
After poking around the net, I discovered you can quit an app by doing the following: Double tap the home button to bring up the multitasking tray or dock. Tap and hold the app you want to quit and remove from the tray. After a second or two, a red badge with a minus symbol will appear on the icon. Tap the minus symbol to quit and remove the app(s) from the multitasking tray. You can also swipe your finger left or right to quit all the current apps altogether.
This is good to know. Even though Apple has optimized how multitasking works in iOS 4 not to drain the battery much, there is no sense leaving apps running in the background if they are not needed.