"The problem is that "FAT" is a very relative term here. Some think anything over 90mm is fat, some think anything under 125mm isn't fat. I think anything over 100mm is fat. "
I have found, just in reading the various magazine Gear 2008 lists (SKI, Skiing, Freeskier, Powder) the definition of "Fat" fluctuates drastically.
For example SKI considers anything in the mid-'80s (usually about 87mm) to be a powder ski, but their reviews of "crazy" powder skis only go up to 110.
Both Skiing and Freeskier start the powder planks at about 110 and then go up, separating the more traditional powder planks from the reverse cambered/tricked out super-fatties.
So it's all relative based on where you ski, conditions, and mindset. For some anything over 78mm will be "fat" for others anything under 110 will be "skinny."
Nobody is right or wrong, it's all what works best for you. Me, I've found I don't cotton to super skinny skis. Then again I'm not a huge carver, more of a sliding, slarving, scrubbing hack who skis half-assed well. So for me skis in the 86-125 range are fine. Really, I just want something that is fun and cuts up the crud and slop when it needs to.
as for this term, first time I ever heard it was on my inaugural visit to Utah. The motel I stayed at in Midvale had a dry erase board where they posted the morning conditions at the resorts. Just about every resort within driving distance was reporting "Packed powder." I distinctly remember asking myself "WTF is Packed Powder?!?" That's a term not tossed around here in Cali. It's either powder (as in freshly dumped that morning) or it ain't.