Apart from the cache volatility bit, which isn't the issue here. Note that SSDs also sport cache(!) If non-volatile cache is what you want, there's a thing called a rechargeable battery that does this without all that tedious fannying about with SSDs and cache copy algorithms.
What you've just described is what most, if not all, the enterprise storage vendors offer, a tiered solution with both rotating disk and SSDs. The more frequently accessed data is held on the SSD's for performance as SSDs have no stroke/latency overhead on Read and Update operations. The disk controller handles the migration of data between the conventional disk and the SSDs transparently.
What would be bloody sexy is if someone implemented a storage driver for PCs to do this automagically, so you could plug in a small SSD and a big disk and present 'em as one device. MS got part way there with ReadyBoost in Vista, but they FAILed big time (surprise) by restricting its use to USB "thumb" drives, so the I/O throughput loss here neatly offsets the seek gains. I reckon this could be the next "must have" on enthusiast mobos, replacing / supplementing RAID.