There are plenty of hard discs out there with comparatively small SSD's built in, designed to cache frequently used stuff on the magnetic portion. Is that what you mean, or is an SSHDD something else?
On the subject of the future of HDDs/SSDs, I agree that SSDs will replace HDDs in almost all circumstances eventually, but it's occurred to me that using a HDD as long-term backup is more sensible than using an SSD. Flash memory is pretty damn resiliant, but eventually current leakage will kill it. But magnetic media doesn't degrade unless moved through a magnetic field (or mechanically mangled so the platters can't spin), so theoretically should keep it's data until it's destroyed in an earthquake, or until the earth's magnetic field has flipped a few times I guess.