Actually, this changed in 3.3. It's up to 5 before before immune, and then reduced effect.
http://www.wowwiki.com/Diminishing_r...#Patch_changes
Quote:
Patch 3.3.0 (2009-12-08): Taunt Diminishing Returns: We've revised the system for diminishing returns on Taunt so that creatures do not become immune to Taunt until after 5 Taunts have landed. The duration of the Taunt effect will be reduced by 35% instead of 50% for each taunt landed. In addition, most creatures in the world will not be affected by Taunt diminishing returns at all. Creatures will only have Taunt diminishing returns if they have been specifically flagged for that behavior based on the design of a given encounter.