Apple has published a document titled "Mac mini (Late 2012) and iMac (Late 2012): About Fusion Drive." The KnowledgeBase article explains what a Fusion Drive is, and includes a collection of frequently asked questions (FAQ) for those looking for more information.
From the article:
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.
TMO's Dave Hamilton penned a detailed article about Fusion Drive following his time with Apple's hands-on session during Tuesday's media event. That article explains how and why the Fusion Drive does what it does, while Apple's FAQ looks at more basic questions.
A selection of those questions:
Do I need to configure Fusion Drive?
When ordered with a Fusion Drive, your iMac or Mac mini is pre-configured.
Can I add a Windows partition?
You can create one additional partition on the hard disk with Fusion Drive. You can create either a Mac OS X partition or a Windows partition.
If I create a hard disk partition is it part of Fusion Drive?
The additional partition is not part of Fusion Drive. The new partition is a separate volume that is physically located on the hard disk drive.
Can a Fusion Drive be mounted on another system in Target Disk Mode?
Yes, but the system attempting to mount the Fusion Drive in Target Disk Mode must have OS X Mountain Lion version 10.8.2 or later. A Fusion Drive will not appear as a Target Disk Mode volume or startup disk on earlier versions of Mac OS X.
There's more in the full article, including details on designating your startup disk and other instructions.