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).
Source:
http://www.modmyi.com/forums/iphone-news/628151-how-force-quit-apps-3-x.html
Source:
http://www.modmyi.com/forums/iphone-news/628151-how-force-quit-apps-3-x.html