I just finished installing an iFixit second hard drive kit and set up a 128GB Crucial M4 and the stock 500GB Toshiba hard drive in a Fusion configuration. Overall, it was easy and successful. I did have one mishap, which was damaging the wires on the IR sensor, but as I had that disabled due to interference with my AppleTV remote anyway, it was no big loss. Effectively there were two Terminal diskutil commands necessary, the one that puts the two disks in one logical volume, and the one that formats the space into the fusion setup. Incredibly easy to do, and Mountain Lion is installing now. Next step is getting the TRIM Enabler going, and then restoring from Time Machine. Right now, my only wish is that I'd bought a 1TB drive, but I can do that later if need be. This was so easy it's kind of ridiculous. If I do get a larger drive, I may get a new HD bracket with IR sensor and be a bit more careful in that step. I'll report back later on any other findings, but so far, it's been a piece of cake!