OS X Mountain Lion does not boot after the creation of a Fusion Drive
It seems that some early 2011 15" Macbook Pro  (8,2) machines have trouble creating a Fusion Drive.

It's being discussed in this MacRumors forum thread: http://forums.macrumors.com/showthread.php?t=1480179&amp;page=16. This is the post that gives more datailed information about the problem http://forums.macrumors.com/showpost.php?p=16321849&amp;postcount=384

It might be a hardware problem, there is no known solution yet.

Update: I have just created a working Fusion Drive on my early 2011 15" 2.3Ghz Macbook Pro (8,2), following the instructions in the answers here: Can I make a DIY Fusion Drive? I have installed Mountain Lion on it.
I used a 250GB Samsung 840 Pro in the hdd bay and placed the original 750GB hdd in place of the optical drive. Maybe the hardware troubles are only present in the 2.0Ghz versions.