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
We've found an answer, finally. Well, kind of. The problem occurred because Xcode 4 places public headers into InstallationBuildProductsLocation folder during build for archive. Apparently, when archiving it sees the headers and tries to put them into archive as well. Changing Public Headers Folder Path of the lib to somewhere outside of InstallationBuildProductsLocation, for example, to $(DSTROOT)/../public_folders and adding this path to Header Search Path solve the problem. This solution doesn't look very elegant, but for us it seems to be the only option. May be you'll find this useful.