Since the new Apple iPhone 4 specs got releasted today by Apple CEO Steve Jobs, we should share what the battery runtime is. From Steve's presentation at the 2010 WWDC Apple Worldwide Developers Conference.

“Now, because we’ve made the battery bigger and the A4 is so good with power management, we’ve improved the battery life! 7 hours 3G talk, 6 hours 3G browsing, 10 hours WiFi browsing, 10 hours music, 40 hours of music, and 300 hours of standby.”

Not sure on how much 3G internet browsing you get out of yoru iPhone 3GS, but I get about 2-3 hours if I'm lucky. Apple claims 6 hours.

Apple Iphone 4 Battery

Source: http://supawesome.com/2010/06/08/iphone-4-%E2%80%93-much-better-battery-life/
http://www.apple.com/iphone/

Cell Phone Battery, Cell Phone Charger, Cell Phone Power Adapter