I am fighting with Xcode 4 workspaces. Currently Xcode 4 wins. Thus, my situation:
I have the workspace with the iOS app project. There is also static library project iO
I also found a solution that works with build and with archive.
In your static library set the Public Headers Folder Path to ../../Headers/YourLib
In your app config set the Header Search Paths to $(BUILT_PRODUCTS_DIR)/../../Headers
In your app you will be able to code #import <YourLib/YourFile.h>
Don't forget the Skip Install = YES
option in your static lib.