I hope it's like wow, permanent stealth when one vanishes. it's beyond cool to sneak past enemy lines and ambush them. Besides sneaking like in Skyrim would not only look ridiculous but also alert any enemy player that you're trying a stealth move on them. Even Guild Wars 2 has a (limited) invisibility time on stealth.
Pretty sure sneaking is like Oblivion and Skyrim, it even has the eye graphic.
Crouch and sneak is controlled by the c key.
Everyone will have access to sneaking and crouch but stealth i can see being class specific: ie Rogue/Ranger.