ComputerWorld had an interview with Apple co-founder Steve Wozniak about solid state disks, a product he's helping to develop in his position as chief scientist at SSD start-up Fusion-io. One of the things he says is that he doesn't believe solid state disks will completely replace hard drives, due to the many tiers of storage for cost efficiency:
Do you see a day when solid state storage will kick all spinning disk out of the data center? No. I don't see it kicking all spinning disks out. In computers we have so many tiers of storage for cost efficiency. Even when you have a hard disk drive it has its own cache built into it. Then we have caching systems in operating systems. Then we have different speeds of memory from your RAM to your L1, L2, L3 caches. This is an in-between one, but I think it's going to be huge - a lot bigger than people think. It cost more money per bit to create NAND flash ... [but] in a lot of places kick out spinning storage. I can see certainly in a netbook you don't want spinning storage. If you're talking 64GB or less, it's less expensive to have flash solid state disk now.
But in a big enterprise-class data center there are huge amounts of data that aren't accessed very often. It's just mathematics. You take stuff that's not accessed very often, it can be accessed slowly. And then you bring it into a faster form of storage when it is being used a lot.
So I can see solid state storage in the enterprise as a type of cache, without the same programming and design structure as cache, but serving that same purpose.