How To: Force Quit Apps on 3.x
Back in the firmware 1.x and 2.x days, there was a simple way to free up memory when an app used to hang - quitting the app "forcefully" by pressing and holding home button for about 10 seconds. Those who upgraded to 3.0 in Beta stages might have missed this useful function - but the good news is that it's back, not exactly the same as before though. Due to the addition of Voice Controls in iPhone 3GS (which are accessed in the same way force-quit used to work) Apple has changed the shortcut, even for 3G and original iPhone users.
Image courtesy TheiPhoneBlog
As discovered by a guy named shutter13, the method now works as follows:
To force quit an application in 3.0, press and hold the sleep/wake button until the slide to power off screen appears, then press and hold the home button until the application quits (about six seconds).
A bit complex, yah - but hey, at least it's back! Another minor change is in the transition when force quitting. Before it used to be like a laggy app screen but now, the app just disappears in like a "poof" transition.
P.S. Even if you don't want to quit an app, use it on Home Screen (not Lock Screen) to free up some memory.