Recently working on an iOS project faced an issue importing project to subversion. All svn commands worked fine from command line, but would not work from Organizer. The issue turned out to be incompatibility between svn client version to the version on subversion server. Xcode was using svn client bundled within Xcode app, while I had later version of the client installed elsewhere available on path.
Here is how you can point Xcode to use latest version of svn.