@NKrader: That makes sense for two harddrive makers; Samsung and Western Digital.
None of the other HDD markers have SSD for the consumer market.

My point is it would be a great value-add that would own competing HDDs, and would allow "green" drives consuming even less power while at the same time boosting performance for general usage.

If the implementation is made in the easiest way i've mentioned above, it won't take much R&D either, and since using this kind of cache is (or can be made) none-fatal to the drive if it fails, there is very little risk of angry customers.