I posted this in Theory and Mechanics initially in case I made a mistake or someone wanted to submit some counter Data (parses, screenshots, etc.). Suggestions/Corrections/Discussion is welcome.
EDIT: Adding this for reference purposes:
EDIT: Based on testing, I think the DR on taunt is 18 seconds. Waiting 17 seconds after taunting, the next taunt was immune. Waiting 19 seconds after taunting, the taunt landed successfully. The last successful taunt was the 3rd taunt, so DR had the debuff duration at 0.75 seconds. I think 18 seconds is a safe bet, despite GC's post on it being 15 seconds.
EDIT: Old WotlK Data (No longer completely correct):
This is a topic of heated debate, but here are what I believe to be the facts:
1. The taunt hit cap against bosses is the spell hit cap of 17%. The hit cap for taunt is not the melee cap. This can easily be tested by putting on 9% melee hit gear and taunting over and over again. I put on enough to have 9.45% melee hit. Doing so I missed on my target.
Remember that the hit cap that applies to you is determined by the level of the mob you are attacking. If you are level 80, then the following applies to you (I need to verify levels 80-82):
Level 80 target => 4% miss
Level 81 target => 5% miss
Level 82 target => 6% miss
Level 83 target => 17% miss (Boss)
Here are the testing results if anyone wants to see them:
2. Taunt chance to hit is increased on the scale of spell hit. I put on 448 hit rating, which is 17.08% spell hit and 13.66% melee hit and then proceeded to taunt a target 283 times. Out of all of those times, there was not a single actual miss. The odds of that happening are about 6.681 in 100000 if taunt increased with melee hit and not spell hit.
I know in 2.3, that melee hit was allowed to increase taunt. However, in WotLK, the two stats are now merged, and only one can increase taunt hit. From my testing it looks like it increases with the scale of spell hit and not melee hit.
To quote Satorri:
Here are the testing results if anyone wants to see them. I had 242 hits, 41 crits, and 150 immunes (notice they are listed as "other" and not "miss").For all tank classes 'Taunt/Growl/Dark Command/Hand of Reckoning' is a spell and takes its caps and rating conversions accordingly, from spell hit chances.
3. Taunt does not double dip. I.E. it is not affected by both spell hit and melee hit at the same time. If you look at my testing in #1, I had 9.45% melee hit. At the same time I had 11.81% spell hit. Combined I would have had 21.26% hit. If taunt double dipped, I wouldn't have missed, but I did. So even though hit rating gives you both stats, only one affects it, and that is spell hit.