But then how would you have the OS install on the Flash portion and the rest on the spinning platters ? Either it would be a cache like the Momentus drive or Apple would have to know in advance what sectors to address specifically in a special partition, meaning a user wise enough could just change this partition from the OS drive to some other folder during installation.
Seems it would be complicating things for no real gain, especially if this was design prior to knowing about the Sandy Bridge bug, which if it exists is the case, seeing how we learned about that bug just weeks ago.
Hardware design doesn't take just a few mere weeks.
I think it would be a cache like Momentus, exactly. The performance gain is huge as compared to software trying to keep track of where things are on two disks, and having to move things from drive to drive by using two SATA channels. As an engineer, no way I would try to engineer it in the OS. (Again, I'm assuming that the two drives appear as one drive to the user. If all they did was treat it like two drives, then that's a completely different story).