Fact
Senior game designer Josh Greenfield (GameSpot, Sept 22): 'Warlock tank, Shaman tank, not quite yet, but never say never.' Shaman tanking is not a supported role at launch; Rockbiter Weapon threat is a leftover original-WoW style option held back by no true taunt and weak mitigation.
Confidence note
Confirmed (dev statement via GameSpot, corroborated by Master of Warcraft)