The release dates for The Walking Dead episode 4: Around Every Corner have been announced. It's coming out in North America on PSN later today. Then, tomorrow, it will be coming out on North American XBLA, and on PC and Mac worldwide.
Release dates for the European market have not yet been announced, nor have release dates for episode 3 or episode 4 for iPad. Not much is known about episode 5 either at this point, but since the entire series is coming out on retail in North America on the 4th of December, the fifth episode will be out sometime before that.
Update: The fourth episode is now out on PC, Mac, and XBLA. Early reports say this is the episode where your previous choices come into play, meaning that the big choice in episode one is no longer the only choice that meaningfully effects gameplay. This is also the episode where the winner of the contest has a character modeled after them who dies and later becomes one of the undead. The runner-up winners are members of the zombie hoarde. So if you won, keep an eye out for yourself. :)