Apple’s posted this Knowledge Base article detailing the new feature:
Fusion Drive is a new storage option for Mac mini (Late 2012) and iMac (Late 2012) computers that combines the performance of Flash storage with the capacity of a hard drive.
Presented as a single volume on your Mac, Fusion Drive automatically and dynamically moves frequently used files to Flash storage for quicker access, while infrequently used items move to the hard disk. As a result you’ll enjoy shorter startup times, and as the system learns how you work you’ll see faster application launches and quicker file access. Fusion Drive manages all this automatically in the background.
Interesting read. Large-capacity SSDs are crazy expensive right now, so Apple seems to want to give users the best of both worlds. Here’s hoping that the rest of the Mac line reaps these benefits as well sooner than later. I’m looking forward to having an SSD in my main machine once I upgrade to a new Mac, probably sometime next year.