there are raid card companies (adaptec and LSI) that are already doing a form of this hybrid, but with arrays. basically they use a single ssd as cache for a HDD array, and the raid controller has intelligent caching that allows for 50x improvement in iops from that array. very smart.

it is called CacheCade from lsi or MaxIQ from adaptec. if they are already doing it for arrays i imagine that it wont be long befor ethey implement it into a single unit.(disk)