One of the biggest limitations of current Ultrabooks is the fact that most only feature a single drive bay which means that users can opt for either a fast, but expensive SSD or a slow and affordable mechanical hard drive. Western Digital new line of SSHD Black "hybrid hard drives" have a thickness of just 5 mm and may provide a solution to this problem.
SSHD stands for Solid State Hard Drive which isn't technically accurate since it is essentially a conventional 500 GB hard drive fused with 24 GB of NAND memory. When asked, WD said that the drive will need drivers to function, which makes us a little sad. Likely, since it'll need drivers, the NAND memory will only be usable as a caching SSD. We would like to see the NAND memory usable as a separate partition or drive so that one could install the OS to the NAND and get full SSD-esque performance for the OS, and have all the data on the slow HDD portion.
No word on availability or pricing, but seen as these drives are primarily aimed at Ultrabooks we can safely assume that these drives will be primarily released to OEMs only.