This Hidden Mickey can be found in California Adventure, in the Redwood Creek Challenge Trail area. Just outside the cave where you "discover your totem" a la Brother Bear is an artificial stream containing artificial rocks to cross on. Most of the "rocks" are simply round, but one is distinctly Mickey-shaped.
Dunno. I'm always on the lookout for them. You can't trust some of the guides--they list too many "Hidden Mickeys" that are just coincidences, or too obvious to be considered hidden.
It's funny...I was going to start such a group myself, then it occurred me to check and see if one already existed. It had been founded about a week prior.