I will admit some of the puzzlement at combining a SSD with an HDD... After all, isn't space at the biggest premium of all in an ultrabook? It's counter-intuitive to then have more than one drive... Even if the "cache SSD" is smaller than a 2.5" drive.
I think we all need to keep in mind what each application needs here. Even for the same user, they're going to need a different amount of storage for each device. For a desktop, I don't think HDDs will ever, ever be fully replaced by SSDs, because they offer easily scalable, cheap storage, that offers vastly superior performance to any and all removable media. For storing terabytes of media, such as high-definition video or massed photo albums, (for both personal and hosting use) a SSD makes no sense: the improved performance goes to waste, and the cost-per-gigabyte (Even amidst the aftermath of the Thai floods) is still way too high.
On the other hand, you have plenty of uses for storage in the rapidly-growing mobile environment: smartphones, tablets, and now ultrabooks are all need storage too, and in their case, they don't need anywhere near as much as a desktop or server. In such a case, the potentially reduced form-factor and power consumption of SSDs can be a boon. The price-per-gigabyte issue can be addressed simply by having the storage space be smaller: after all, given that smartphones and tablets lack an HD screen, they obviously won't be playing HD video. Few (if any) people are complaining "I wish my Android/iPhone had 2TB of storage;" they're generally happy with the 16-64GB size range available today.
Hence, for an application like an ultrabook, two drives makes little sense. And given the push for extreme slimness, and the bulkiness of even the thinnest consumer HDDs, it'd be best to just ditch HDDs entirely; after all, that's what the current MacBook Air ultrabooks do, and most of the whole point of other companies' ultrabooks is to take the market from Apple.
Also, as I recall some of the growth in SSD sales is still coming from the price spike in HDDs, which, for some users who'd been on the fence, would give them just enough of a push to take the plunge on SSDs.
[citation][nom]drwho1[/nom]This "cache SSD's" should be embedded into Motherboards with a capacity of at least 120GB in the near future.[/citation]
This begs the question of, "why?" For some that's an unnecessary expense; flash memory cells run in the neighborhood of $1US per gigabyte, so you're basically adding $100US+ onto the price of a motherboard. And for others, that 120 GB may be woefully insufficient.
So what's the solution? Perhaps motherboards should have a plug-in slot for flash memory, so users can buy individual modules and plug in whatever ones they want- oh wait we already have that: it's called SATA.