I've developed application for the original iPhone, the 3G and the 3GS, now I need to develop an application for the iPhone 4. What is the difference between developing an application for iPhone 3GS and the iPhone 4?
The most noticeable change is the Retina Display and you should include @2x images in your project to support this. Other than that unless you plan on taking advantage of new hardware (gyroscope for example) or the graphics and cpu improvements, there is very little difference. Also you should reference the SDK Compatibility Guide for tips on maintaining backwards compatibility and/or supporting features available only on certain devices.