Here's what I did:
- in Terminal cd to the project.workspace directory
svn -v status
revealed this file was under version control (M):M 655 654 user xcuserdata/user.xcuserdatad/UserInterfaceState.xcuserstate
svn delete --keep-local --force xcuserdata/user.xcuserdatad/UserInterfaceState.xcuserstate
That fixed it. At that point Cornerstone showed that it was ignoring the file. And Xcode behaved properly too.