First off, whoever is coming up with component names for Apple deserves a raise. "Retina" display. "Fusion Drive." That's pure gold. I like the idea of having storage that sounds as if it could provide backup propulsion for the Starship Enterprise (Next Generation version, please). "Engage the Fusion Drive, Mr. Cook."
Anyway, from what I've seen so far, the Fusion Drive looks like a pretty sweet idea - combine the speed of SSD and the cheap capacity of a HDD to get, er, speed and capacity. That's why I'm not a writer for Apple, folks. Will these be compatible with older Macs that didn't ship with Fusion Drives installed, or is it new Mac only?