I notice that the screen resolution for the iPhone 3+ is 240x480 and the iPhone 4 is 640x960 ...
Does this mean that you'd want to have 2 sets of graphics that utilize the extra resolution? My first thought on that would be that the higher the resolution, the smaller the graphics would appear.
You do need two sets of graphics. The older iPhones are actually 320x480. In general you can provide only a low resolution graphic if you'd like, and it will be automatically upscaled, but going only one high resolution graphic doesn't work automatically. The handling of the second graphic is done automatically, based on filename, so you would have one file named mygraphic.png, and a second named [email protected]. Since the new screen is exactly double the resolution of the old screen, you don't need to do anything special to get it working beyond having the second file in place.