I have designed a UI for the iPhone 4S. What will be the application appearance be if I use the same images and UI for the iPhone3? If I run the same application in iPhone3 and iPhone 4S, what will be the outcome?
Nothing should happen. The iphone 4 has a higher pitch screen but doesn't usually effect the number of pixels in an image/UI however I would think using high res UI design would slow down the iPhone 3. the best way to check is design a background image or UI element and see how they look on each iphone.
This is probably more of a problem if you are talking about OS based icons e.g. for the home screen as already mentioned the 4S has a higher resolution screen than the 3 and therefore up-scaling may result in pixelation. If they are code based rather than pixel then its probably again not a problem.
If it does end up a problem targeting device version should be possible.