For reasons stated in other answers to this question, you can’t inherit values easily.
I recommend defining your settings in cascade. Let’s assume APP is your project prefix and make this simple defining only a few CFLAGS:
platform.xcconfig:
APP_PLATFORM_CFLAGS = -DMAS=1
project.xcconfig:
#include "platform.xcconfig"
APP_PROJECT_CFLAGS = -DBETA=1
target-one.xcconfig:
#include "project.xcconfig"
APP_TARGET_CFLAGS = -DSUPER_COOL=1
#include "merge.xcconfig"
target-two.xcconfig:
#include "project.xcconfig"
APP_TARGET_CFLAGS = -DULTRA_COOL=1
#include "merge.xcconfig"
merge.xcconfig:
OTHER_CFLAGS = $(inherited) $(APP_PLATFORM_CFLAGS) $(APP_PROJECT_CFLAGS) $(APP_TARGET_CFLAGS)
Then, you’ll base each of your targets build configurations on target-xxx.xcconfig. A real project will use more complex setups, using a configuration file for the project and a different one for the target, but you get the idea.
Also, remember that $(inherited) refers to higher level in the hierarchy, not earlier. For instance, it inherits from Project level at Target level. Not sure if this apply to Xcode 4 too.
This is a simplification of GTM, go there to learn more.