I've been playing a level 60 barb and I just finished hell difficulty today and attempted inferno, (didn't go to well). So I decided to go back and play some more of act 4 in hell. I can join any quest in hell and whenever I leave, that specific quest is unlocked in inferno difficulty, no matter what act or quest it is. I haven't reach the skele king yet and I've been getting the option to join a game in act 4 inferno if I just left a game with that quest. Now when I make a game and leave, that option is gone, I can only join a game as far as I progressed. But once I join a public game, that inferno quest is unlocked.