I missed this last week, but SemiAccurate pointed me to an intriguing article over at Bit-Tech that details Marvell's plans to allow users to roll their own hybrid storage configurations using free software dubbed HyperHDD. You'll need one of Marvell's 6Gbps 88SE9128 storage controllers, which can be found on quite a good number of high-end and mid-range motherboards these days, plus a mechanical hard drive and an SSD. Plug the drives into the Marvell controller, and the software will take over and use the solid-state drive to cache frequently-accessed files.
The concept looks similar to SilverStone's overly capitalized HDDBOOST scheme, which uses an SSD to mirror a mechanical hard drive's "front-end data." Requests for that data are serviced by the SSD, while others are passed along to the mechanical drive. However, HDDBOOST doesn't appear to populate the SSD intelligently based on user access patterns, and it forces both drives to share a single 3Gbps Serial ATA interface. The SSD can't be used to cache incoming writes, either.
There's no word on whether Marvell's approach will allow for write caching, but I wouldn't hold my breath. Even Seagate's Momentus XT hybrid drive uses its flash memory exclusively as a read cache. That's really a shame, because intelligently managed flash could make one heck of a general-purpose cache sitting between a system's speedy main memory and vast mechanical storage. Microsoft moved a little in that direction when it developed Vista's ReadyDrive support for hybrid hard drives, but the feature seems to have been abandoned by the few drive makers who showed initial interest. With Windows already speculatively caching data in memory with SuperFetch and thumb drives with ReadyBoost, tackling discrete SSDs would seem to be the next logical step.