Apple introduces 'Fusion Drive' as a build-to-order option for the new iMac
 

So you want the performance of a new-fangled SSD with the extra storage of a good 'ole HDD? Apple has you covered with Fusion Drive for its new iMac. Unlike existing standalone hybrid drives the company is apparently "fusing" together a separate SSD and HDD with custom software in Mountain Lion. Better yet, the OS and all the pre-installed applications live on the SSD by default, while your documents and media reside on the HDD. This sounds similar to the software RAID functionality in OS X, but cranked to the next level. Stay tuned for details as we find out more.

http://www.engadget.com/2012/10/23/apple-fusion-drive/