The Scratch API will likely have major changes with the release of Scratch 2.0. The new platform has a "different (and much better) authentication system", according to Lightnin. There will also be a CAS (Central Authentication System). However an API will also be available, as CAS requires custom server software to be installed.  Nothing else is currently known about it.
See: Scratch 2.0 Alpha API Documentation (by nXIII)