Hybrid cars solved the problems associated with electric cars and fuel guzzling engines. By bringing both of the technologies together, Hybrid cars can function on gasoline and still save costs by switching to an electric engine when possible.
A similar problem in computing industry is forcing storage manufacturers to work on a new kind of hybrid storage device called a Hybrid Drive. This device is a result of combination of the technologies behind regular disk based drive and the faster USB drives on your keychains. This combination provides it with high speed data access and cheap-per-byte pricing in the same storage device.
This concept isn't new, and if you have worked with storage devices you will remember that most high end RAID devices already have an internal cache which does something similar. Infact most Operating systems, including Windows, Linux has Solaris have builtin file cache too. But most of these devices don't use non-volatile Solid state (flash) which forces the cache to be destroyed everytime the Operating system is restarted. Solid state cache within the Harddrives can not only survive reboots (if non-volatile memory is used), it can also reduce the dependency on third-party caching software and hardwares which can introduce its own set of problems.
One thing to note is that though overall i/o speed will improve, the Solid state storage within HDDs will probably never completely replace in-memory(RAM) cache.
Though the technology behind this has existed for a while with a few very expensive implementations, its not until now, due to dropping solid state prices, that we might have a real chance at seeing this in action inside our home computers.