I played a nightblade and templar through 2 beta weekends, and as long as I got off an attack from behind or the side while sneaking, it would apply a short stun and do bonus damage. Playing my nightblade yesterday, I tried multiple times, but not once did I get bonus damage nor did it stun. It looked like I got bonus damage when using teleport strike from stealth, but I can't say for sure, will test more after work today.
I do hope this is a bug, and not working as intended? Cause that would render a couple of nightblade abilities pointless/useless. It would also seriously hamper melee stealth gameplay.