Well smokey must have an area where he has power and can't go outside of. We've never seen him on the beach, and I guess that's why he was tearing up trees in the jungle on the night of the pilot, he was spitting mad he couldn't play with the newcomers. I'm guessing he must stay in a an area around where the vents are located on the map, perhaps he gets his power from there, and teh magnetism to remian a somewhat cohesive shape.
The Others just may be still inside the fence, and ben was shaking because he had to travel outside the fence until he got past the ashen border of the shack.