Nice article; good quick overview.
As for your wants, I should note that there was something in the Xcode release notes about subprojects. I believe that you can now add a project to another project and have your targets in the enclosing project depend on targets in the subproject.
Also, there are unit testing frameworks available which do just this. I don't know much about them, but OCUnit and ObjCUnit (I think) are both just exactly like that. As to whether they work in Xcode yet or not, I dunno.