How can I use .xcconfig files in Xcode 4?
Pick File | New, choose Other and "Configuration Settings File", this gets you a new .xcconfig file.
Then click on the project on the upper left in the file browser (the same place where you get to build settings), click on the project again in the second-to-left panel, and click on the Info tab at the top of the inner panel.
There, you can choose "Based on Configuration File" for each of your targets for each of your configurations (debug, release, etc.)
Hope this helps!
Great answer from Mike Kale. If the description was not clear enough, here is a screenshot.
