Here's a recipe I found from the new Marlboro Country Cookbook. With a few tweaks, it look pretty good.

Brown meat and add broth. Cover and slow boil for 60 minutes until tender. Scrape spines off nopalitos and cut in 2 X .5" strips. Cook in boiling salted water for 10 minutes until tender. Husk and chop tomatillos. Saute onion and garlic in oil until tender. Add tomatillos stirring until soft. Add cumin, salt and cayenne. Add tomatillos to beef. Slow boil for 30 minutes. Add nopalitos, chiles and cilantro just before serving.

Pork roast would be a good alternative here instead of beef. Either would benefit from a few hours on the smoker first.