Inspired by an Jinx's article about building your own fusion drive I've decided to take it one step further and brew my own with full OS X installation on it.
First, I needed to put two drives in my Mac. I already had a Crucial M4 SSD and 500GB HD so I a bought hard drive caddy and put my 500GB Momentus XT in it.
That was the easy part. Now how to make them work? In Jinx's article you can read that GUI Disk Utility does not offer needed functionality. It's available only in command line version, diskutil. But from my previous installation I remembered that there is access to terminal in OS X's recovery mode.
It was downhill from there. Create a logical volume group, get the UUID and create the volume. The OS X Installer recognized the volume and installed nicely. After the whole process was over, I did tests similar to those that Jinx mentioned and the drive does, indeed, behave like a fusion drive - files used more often end up on the SSD and less used are shuffled to the HDD.
You can see screenshots and more information here:
Mountain Lion on home made Fusion drive
[kirkmc adds: I don't usually run hints that don't explain things, but the first article linked above goes into great detail. I'm tempted to try this out, as my Mac mini has both an SSD and a 750 GB HD, but I'd need to move all the files - my music collection - from the HD to an external drive. I might try and get to it this week.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20121104184756656