I have 4G only and my project has GUITexture “buttons” on the screen. Resolution is set on “Native res…”. If I run the game on iphone4 , everything is ok. My question is, if I run it on 3GS, will be every button on the same position and size, or it will “hide” behind the screen borders because of resolution? If so, how to solve that? I have no 3GS right now, so I can’t test it. 
Your best bet is to make the game resolution 320 X 480, or 480 X 320 the iPhone 4 can handle these resolutions fine, and the game will still look good and you wont have to worry about the older devices not supporting the retina display. Then i’d concentrate on a retina version after…
if you develop for 480x320 be sure and have you target resolution set to “Standard (Low or Medium resolution)”. I set mine to “Native” and when i released my game it the gui was all messed up on iphone 4. i don’t know how it made it through review. so i was stuck waiting a week for the fix to get published.
OK, thx. I would like to make iphone4 game only, but don’t know how to deal with it. Because if will be possible to download it for 3GS, there will be this problem with resolution. I can make one version for 2G, 3G and 3GS and next version for 4G only, if is it possible… but I think arm7 is not the right way, because 3GS can handle it too. Any idea?