I made a normal mob spawner with dispensers on both ends of it. I put some redstone stuff around it to make the dispensers release water when enough mobs have spawned, and when done the dispensers will suck in the water pictures right here. This is 1.3.1 I'm playing.
Hm, nice! I'm thinking of doing this for a gold farm. Zombie pigmen spawn rarely around nether portals so I was think about putting 50 in one area, then flushing them to a kill zone.