With iPhone 1.x and 2.x, you could hold down the Home button to "force quit" an application (kill the process) and return to the Home Screen. With iPhone 3.0, however, holding down the Home button now activates Voice Control on the iPhone 3G S.
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).
Yes, this does indeed mean holding down Home is now useless on iPhone 1.x and 2.x, but that most likely will only affect jailbreakers and others who have specific reason for not upgrading. Everyone else gets a slightly more complicated, but still functional, Force Quit.