Flash memory is being used as an acceleration layer in many enterprise applications and recent announcements by Macronix indicate that the write life endurance for flash memory cells can be increased considerably by locally exposing NAND flash memory cells to a high heat (about 800 C).
Write caching would store data to be written on the HDD in flash memory initially until enough data was in the flash cache to do an efficient write to the HDD disks.
By building heating into the flash memory design, Macronix thinks that flash memory cells could last 100 M write cycles rather than the 10 thousand or so cycles possible today with MLC flash memory.
SSDs using NAND flash memory are capable of very fast read and write speeds and thus can make immediate use of such fast interfaces (whereas it takes many SAS HDDs working together to result in data rates even approaching there speeds).