It looks like good news for Aperture users. There's a post on ArsTechnica relating that Apple's new Fusion drive does indeed go well beyond simple caching and more importantly, operates at the block level. For Aperture users, this means that a Fusion drive will keep only the most used parts of an Aperture library on the fast SSD drive, whilst the rest of the Aperture library can reside on the 'other part' of the Fusion drive (a normal HDD).
The best news though, is that it appears that the Fusion drive will be accessible via any SSD/HDD set up. IOW, grow your own without having to buy a new iMac.