Unfortunately Apple pulled the plug on their brief flirtation with ZFS just before Snow Leopard (10.6) was released, IIRC. See these three articles
for that story. I should point out that ZFS is apparently useable for "secondary storage," just not by default and not on your system volumes, etc.
Fusion Drive is something different; Apple's basically using software to weld separate SSD and spinning HDD into something better than Seagate's hybrid Momentus SSD+HDD devices
. More details from AnandTech
. My skimming of the explanations make Apple's approach sound more like hierarchical storage management (HSM) applied to the two devices, versus pure caching where the capacity of the cache is "lost" to the end-user. IOW with 1TB of HDD and 128MB of SSD, Apple gives you more like 1.12TB useable storage whereas a hybrid self-contained device would give you 1TB.