Solid-state drives  are slowly taking over the PC data storage market--and for good reasons, with their reduced power requirement, resistance to knocks and potentially faster access times than regular hard drives. And the next step in their development will integration of the chips right into the PC, instead of just being used as add-ons.
Solid state drives, whether 1.8-inch or 3.5-inch types are essentially NAND flash chips grouped together on a circuit board with an input-output controller, bundled into a casing that fits the standard size and equipped with a connector that wires them into your PC.
But the SSD components are essentially just a bunch of surface-mount semiconductors, just like most other chips inside the computer. Why bother to separate them? It's just a convention.
Current SSDs are generally connected with Serial ATA cables, a modern 8-pin serial data replacement to the older parallel AT Attachment connectors that required bulky 80-pin wires and have a legacy going back to 1986. But though it's a faster connection than PATA, it's still a legacy standard designed for hard drives. By integrating the SSD NAND chips into a PC design from the get-go, you'd abolish the need for the legacy connector with its wires and sockets, and remove the need to use an old-fashioned and potential non-optimal input-output standard. Instead of a peripheral to the PC the drive would be seen as an integral part of the memory, acting as "long term memory." It could even utilize the fast access offered by the modern PCI-express bus, improving stored data access times and speeding up the operation of the entire machine.
Hard drives traditionally have occasional failures that require you to whisk one out of a PC and replace it with a new unit--simply a function of their high-speed moving parts design. But there's significantly less need to do that with SSDs. Sure, they have a designed-in lifespan, largely due to the number of read-writes that each cell of memory can put up with--but that lifespan is pretty long, and probably equates to the useful lifespan of the machine's they're placed in nowadays.
Integrating the components of an SSD directly onto the motherboard brings a huge number of advantages: There'd be a reduction in size and weight, since you wouldn't need a drive casing--useful for mobile computers. A small cost saving would run with that too, and it would let designers optimize the computer's design how they saw fit: no need for access to a drive slot, and freer choice on how to organize the circuit layouts. Apple's exploiting exactly these advantages  with the built-in, user non-serviceable battery on the new 17-inch MacBook Pro.
Some analysis by market researchers In-Stat's has placed SSD costs currently at $2 to $3.45 per gig, in competition with around $0.38 for a hard drive, but prices of solid-state systems have been dropping 60% year on year. Much research is going on to both increase the capacity of SSDs and reduce their price: Seagate, for example, is throwing $100 million  at the production of new SSDs.
The next step is to abandon legacy drive formats left over from the hard drive era, and integrate NAND drive chips directly into our computers: With reduced access times to data, reduced power consumption and longer battery life for portable PCs it's in the designers, and consumer's best interest.