iPhone 3Gs vs iPhone 3G

Main differences in Technical Specifications and functionalities include:

Processor Speed:
iPhone 3G: 412MHz
iPhone 3GS: 600MHz

RAM Capacity:
iPhone 3G: 128MB
iPhone 3GS: 256MB

Camera:
iPhone 3G: 2-megapixel camera
iPhone 3GS: 3-megapixel camera with Auto-Focus and Video recording and editing

Voice Control:
iPhone 3G: Only third party apps that allow some voice control functions
iPhone 3GS: Native (build it) voice control software for dialing Contacts and music track name identification and song selection

Compass:
iPhone 3G: No Compass
iPhone 3GS: built-in digital compass that automatically reorients maps and street views

For a full list of iPhone 3G S features please visit Apple at:
http://www.apple.com/au/iphone/iphone-3g-s/

Or the following page for a side by side comparison:
http://www.iphoneheat.com/2009/06/iphone-3g-and-iphone-3g-s-comparison/