If you've been lusting after a Fusion Drive but own an older Mac, a Fusion-like solution may not be entirely out of your reach.
Posts at the jollyjinx tumblr detail how the eponymous hacker built his own Fusion-style SSD/Hard Drive hybrid using features built directly into OS X. Lee Hutchinson over at Ars Technica explains the technology behind this, discussing how Core Storage (first introduced in Lion) can tie two devices together into a single logical volume.
If you're interested in the fundamentals that power Fusion, both Hutchinson's post and jollyjinx's write-ups (Fusion drive on older Macs, More on BYO Fusion Drive, Fusion Drive Loose Ends) offer absolutely fascinating reads.
As for me, I'm just going to use the Apple-built version that should arrive on November 6, according to my shipping statement. If you have a spare Mac with an internal SSD and end up trying this all out, please let us know how it worked for you.