[villiros]

From what I can figure out, Auto-Save and Versions are pretty-much built-in. Assuming an application that uses standard Cocoa approaches to document management, it just needs to declare its support for the auto-save functionality, and pretty much everything else will be handled by the system libraries. Fairly trivial.

Resume is more involved, however: the application is responsible for (de)serializing its state. Again, for a document-based application that doesn't stray far from the "recommended" path, it should be fairly simple to implement. It's just a matter of the document and UI being auto-saved on termination, which requires little work from the developer. One very interesting side-effect of the Resume feature is that it removes the need for applications to stay running: the system can silently terminate unused applications, and bring them back if they're needed again. What's really exciting about this, is that it's turning applications into something that is "always there".