Officially, Fusion Drive will launch with the new iMac but it is already possible to activate it if you have a Mac with an HDD and an SSD. The whole process is described on this page:
As a reminder, Fusion Drive allows to create one logical storage volume comprising the SSD and the HDD. Data is first written on the SSD and once it is full, some data will be transferred to the HDD. Wile doing so, Mac OS will keep 4GB of free space on the SSD to be used as cache before transfer to the HDD.
This solution has the advantage of taking reserving only 4 GB of the total storage space for cache. There are however some drawbacks as the acceleration provided by the cache is limited to these 4 GB, and in case one of the drives fails, the data of the entire volume, including on the data stored on the other drive, is lost. (The only solution has always been and will always remain frequent backups)
In addition, this solution is not as fast as a single large SSD, which is not much more expensive since the price per GB of Apple's SSDs is 3 to 4 times higher than on the rest of the market.
ZDNet published a video showing the new Fusion Drive on a Mac mini with an SSD and an HDD.
As you can see, it is a lot faster than on a Mac with the HDD alone.