If I create a iPhone tab-style application in XCode (I use 4.2), what should I do after the project is created to configure it to design for iPhone 4 (high res) screen vs the 3GS and lower (low-res) screen?
I assume it defaults to the lower-res screen? Since these First/SecondViewController.xib files don’t look very high res to me.
You don’t have to do very much. Built-in UI elements will automatically be high-res when the app is run on a device with a retina display.
As for any images you use, you can include a regular and higher-res (i.e., the same image scaled with double height and double width), and the appropriate one will automatically be used. To name your images, if you want to use an image named
foo.png, use that as the name for the lower-res version, and then save the higher-res version asfoo@2x.png, and it’ll just work.