I made this today. It was fantastic.
The recipe is from Tyler Cowen (economist, ethnic food expert, and blogger. Check out his homepage.)
I modified the recipe slightly. At April's suggestion I used poblano chilies. The pepitas I used never popped when I cooked them. I just cooked them to a darker color over medium high heat in a little oil. I used my cast iron pan for all the cooking.
Addendum: The sauce needs about 1 tsp salt.