I _think_ the new iMac has Apple's SSD implementation like the rPros and Air (i.e., the no-case SIMM like components), and then a space for a 2.5" conventional HDD.
I'm thinking the Fusion config is SSD "cards" + a separate HDD (then the presence of both and the OS is set to optimize the storage allocation).
Makes sense. Cards or mSATA perhaps? I am holding a Crucial 64GB mSATA, and it is not much bigger than an SD Card... The new Drobo 5D is using mSATA to bring this sort of hybrid performance to their new RAID storage.