Detail
Wowhead's Sept 16 datamine lists Embrace of the Viper as a normal 5-piece set (2pc Int, 3pc AP, 4pc Embrace of the Viper, 5pc Dream Venom) with no form mentioned; Icy Veins and Master of Warcraft say wearing the full Wailing Caverns set turns the Druid into a serpent. Most likely the 4pc/5pc effect is the transformation, but Blizzard has not said which piece count triggers it.