I'm sure it carries risks, as it's not officially supported. Have a bootable clone of your drive on hand in case it all goes south.
That said, a few weeks ago I did exactly what you describe (with a 2011 Mac Mini, a hard drive caddy kit from Other World Computing, a 500GB HDD and a 120GB SSD). The physical installation was a bit of work, but doable, and the command line stuff was quick and easy. It's so much of a speed improvement, it's like I bought a new computer. Seriously. And from what I can glean, the magic "fusion" data migration between the two physical drives is taking place. Everything has been running rock solid, and the only time I've had to reboot was when I accidentally unplugged my Mac with my foot under my desk (idiot).
I followed the instructions here:
http://blog.macsales.com/15617-creating-your-own-fusion-drive
Have seen sporadic reports (on these forums) of certain machines having trouble with a DIY fusion drive. I'd google around as much as you can before taking the plunge. But as long as you have good backups, you can always fall back to manually separating stuff between the SSD and HDD.