I don't know about the slims, but the fats will accept any laptop size SATA hard drive.
There are 3 differently shaped PS3 models out there (four models if the 12GB is different, which I've never seen).
1. Original (Also in a slightly different PS2-playing version with extra ports).
2. Slim (my favorite, and probably the one you are referring to)
3. Superslim (I guess thats what they call this one) that originated with some Uncharted game and a 500GB hard drive. This model looks like a 2nd model Sega CD with its pop-up door (well sliding door on the PS3) that looks and feels like a cheap toy.
Anyway, I've never messed with #3 (which I assume the 12gb is as well), but both 1 and 2 have easily replacable laptop hard drives.