I'm still looking over the postings around Computex 2011 (link) and I found something else very interesting - a Hybrid PCIe drive. OCZ was showing what they called "RevoDrive Hybrid" that is a PCIe SSD and an HD, all on the same PCIe card as shown below.
OCZ RevoHybrid PCIe Drive (from www.anandtech.com)
The drive is very interesting because it takes the new OCZ Revodrive 3 PCIe SSD and uses it as a cache for a 2.5" HD (500GB or 1TB) that is on the card.
The approach in this drive is a bit different from the normal hybrid drive. In the first generation of hybrid drives a little bit of SSD storage was added to a normal hard drive to act as a secondary cache (don't forget the RAM cache on the drive). With the right firmware it was hoped that the SSD bit would prove useful as a cache because it was mugh larger than the RAM cache but reasonably cost-effective. I didn't see too many benchmarks that showed it was useful overall, although for some read-only patterns it wasn't too bad.
For this new hybrid approach, OCZ took a very fast SSD on the PCIe bus and coupled it with a backing store that is a hard drive. The way I like to think about is that I didn't add SSD as a cache for the hard drive, but instead, I added a hard drive as a backing cache for the SSD (a little counter-intuitive). In this case you think of the SSD as the primary storage and the hard drive as a simple backing store that allows you to accumulate data.