Like posted before more and more guys are doing it. I think it's more than just moving a foot back into position. They hold it out and near the ground for a good length of time. And watch closely, you can see them move their foot up and down. Yes, it's always on left turns and not rights. But, remember the rear brake is on the right.
I tend to see Rossi do it more while passing WAY on the inside. Pedrosa too. Nickey rides flat track so I guess he has an excuse.
And as someone posted above, it's done at certain tracks in specific corners, but not all.