XCode's UI was heavily refactored in XCode 4. I'm working through some examples from Big Nerd Ranch's "Cocoa Programming for Mac OSX (3rd Ed)" and hit a stumbling block with the RaiseMan exercise. I thought I'd share the solution.
In that exercise you'll associate a file extension (".rsmn") with files your app saves. Your app will auto-save at this point, and should be able to reopen files you previously saved or auto-saved.
I followed the directions in the book and took into account XCode's new UI, but that was a recipe for massive fail-sauce. After debugging things, reading through docs on particular deprecated functions, the solution was simple. "Do not use an identifier." I'm sure identifiers are useful, but using one broke the heck out of my project.
If you've done this, go back to your Info screen for the RaiseMan target and delete all Document Types, Exported UTIs and Imported UTIs you've created, then rebuild & run. Your app should work again. Now go back to the Info screen for your RaiseMan target, add a document and populate only the Name, Class, Extension and Icon fields. Do not populate the identifier.
After completing this step, your app will both save and open files with the ".rsmn" extension.