Given that hybrid hard drives like the Momentus® XT never really took off (there is only on manufacturer, so tier 1 OEMs will not use them, plus current drives and only cached reads anyway), it looks like there are limited options for SSD hard drive caching, an add in SATA card or a software solution.
Unfortunately, none of the current options can make normal hard drives perform like SSD's. At best you get performance somewhere between SSD & HD and at worst you get performance which is even lower than the HD on it's own.
On the plus side, if you are upgrading an old SSD then using that old SSD to cache your large storage drive could be an excellent option. SSD caching seems to benefit much less from newer, faster SSDs so you get most of the caching benefit from older, relatively slow SSDs.
Add-in SATA cards
One option is an add-in SATA card which can provide this functionality. I believe there are enterprise level solutions for this, but as they are well out of my price range I haven't researched them. The HighPoint RocketHybrid 1220 is much more affordable though, and there is a nice article about this on Tom's Hardware Guide.
My experience with the Add-in SATA card option.
Having seen the Add-in SATA card option in action, I have to say that I'm not impressed with the Marvell 88SE9130 based card that I bought. HyperDuo was incredibly unreliable, the software kept crashing and the performance was rarely higher than the underlying hard drive on its own, even after hours of optimisation.
Even just using the card as a 6Gbps SATA III port resulted in worse performance with an Adata S511 than just using the on-board 3Gbps SATA II ports.
Intel Smart Response Technology (may require a motherboard upgrade)
Another option is to upgrade to a motherboard with an Intel Z68 chipset, or newer chipset that supports Intel Smart Response Technology. Again, THG has a nice article providing an overview of what we can expect of the SSD caching capabilities of this chipset.
My experience with the Intel SRT option.
Having recently had the opportunity to play with a Z68 based motherboard, I'm even less impressed with Intel Smart Response Technology than I was with HyperDuo!
Smart Response Technology can only cache a Windows boot drive, so you can't have an SSD Boot drive and then use SRT to cache another drive in your system. Even then, you have to install Windows on the hard drive, install drivers, then SRT and then add the SSD. At this point, if you are lucky then you will see a new "Accelerate" tab on RST application. If not then you may have to start the whole process from scratch to try and get it to work.
Alternative software caching options
The latest option is a non-Intel software caching solution, like Dataplex, which is bundled with the OCZ Synapse SSD. Unfortunately, this solution (like Intels SRT) currently only supports caching a single boot drive, so the only benefit is that it doesn't require a Z68 motherboard.
Unfortunately, I have yet to see a review from a site whose methodology I trust, so I can't tell how this option compares with SRT or the add-in card option.