Mod or someone PM me and I'll tell you.
It's actually funny how easy it is.
(Not actually reproduced it yet, but I've not finished act 1 yet I am in the crater now (think it's act 4)... Also know what I did for this to be unlocked)
Edit:
*** Make sure you're in a public inferno game before following these steps
Step 1, have someone join act -- in nightmare mode (be a quest or 2 BEHIND the one you want to unlock in inferno)
Step 2, request invite from your friend
Step 3, they accept and you join their game
Step 4, run them through 2 or 3 quests of the act (I did arreat crater - Azmodan on nightmare for this guy)
Step 5, When you kill a boss and finish quest click leave game (Not exit game)
Step 6, Go to quest select and you should have it unlocked in inferno mode.
Hope this helps.