Similar to my Execute report, I've had very limited opportunity to test this.
However, it seems that Taunt does not work if you use it on a mob that is casting a spell.
During an AV game, I taunted Lokholar the Ice Lord while he was casting a frostbolt and after his cast finished, he still had aggro on the person he casted the spell on, while my taunt was still on the boss as a debuff.
There was no mention of a taunt resist in my combat log either. This bug occured twice in total and I haven't had the opportunity to test it more in a PvP test.