WD Shows Off SSHD Hybrid 5mm Hard Drives
Western Digital is working on a 'hybrid' hard drive, a big hard drive mated with a little SSD
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.

But with MORE flash
The driver requirement honestly sucks because it may not work right with other operating systems.
I like the 5mm height.
No. They're actually quite different. Seagate's Momentus XT (I've got one) is not bad by any means (especially the newest models!), but it uses a much simpler caching method. The advantage of Seagate's approach is that it is done entirely on-drive and it is software-agnostic - meaning it works on any OS, and requires no drivers. In fact the OS will just think it is a regular mechanical HDD and treat it as such.
The disadvantage is that the Momentus XT is not as good at knowing WHAT to cache. A driver-based solution will be similar to Intel's SRT caching system. It will be smarter, faster, and has a lot more cache to play with. In fact, given how good some of the caching systems are, I must disagree with the author's assertion that you'd get better performance by having complete manual control over the drive.
Think about it. If you install Windows on the tiny SSD, you've got a lot of files in that installation that aren't accessed regularly. That's a waste of precious cache, and leaves less room to cache installed programs! So instead of automatically caching most of your software's commonly accessed files, you're suggesting that we manually cache ALL of the files of a more limited number of programs? That leaves the rest to run ENTIRELY on the mechanical side.
Just let the caching system learn what it needs to keep in NAND and what to keep on the mechanical bit, and in regular daily use the system will have better overall performance. It might not be as fast at a particular task, but it will be better across the board. It also greatly simplifies space management. You could shove this drive in your parent's laptop, install the drivers, and they would be able to enjoy the speed boost without worrying about capacity.
If you really want no compromise, you either have to get a very large and expensive SSD, or get a larger laptop with dual bays. I actually lean towards the latter, myself, but most people don't have two bays, don't have tons of cash to burn, and have at least some space concerns.
(okay, technically you could install it if you were really selective about what apps, patches and services you ran)
So in a 500GB model atleast 64GB would be NAND. Who cares if it drives up the price somewhat, a vastly superior experience would more than make up for the extra cost.
So you should put the term "SSHD" in quotes, not "hybrid" and "hybrid hard drive".
But it needs drivers
I'll take an OS independent hybrid drive any day.
24GB of flash is nice but since this drive is aimed at ultrabooks it is probably just 5400RPM.
Solid State Hybrid Drive
You're right. I'll just shove a few 3.5" drives in my Ultrabook and I'll be all set. /sarcasm