In the 1.x and 2.x days, when an application hanged up, you would just press the home button for a while and quit to the home screen. That was the only way to force-quit then and free the memory. But with the introduction of Voice Control on iPhone 3GS, the old method rather starts voice control or simply does nothing on 2G and 3G iPhone.
Someone at EveythingiCafe has found the new method to force-quit on 3.x.
To force quitted an application in 3.0, / iPhone 3G S 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)
The process just got much more complex, but hey, there’s a positive side to it. Now, instead of getting a laggy transition while force quitting, there’s a smooth poof like transition. I don’t think this’d stop anyone from upgrading. :p.