The rumor yesterday indicated it would be for the OS and some of the most common applications used. Not sure if they are "pre-picked" apps (like Safari and iTunes), or if it is based on your own personal usage. It would be cool if you could allocate an App to load onto the SSD... like Photoshop or Indesign which is like watching grass grow to launch.
I'm sure it will include everything that is part of a default OSX install such as safari and itunes. Of course itunes music and such would be on the secondary drive.
It wouldn't surprise me if they had an option to replace the secondary mechanical drive with a large SSD for those who want it.