Heartbeat resist seems to be functioning as it should be in TBC (all long duration CC's limited to 12 seconds maximum)
How I think it functions currently:
Either one of the two patches below, although I think it is 2.2.0 version. Haven't simulated enough CC to get a reasonable sample.
Patch 2.2.0 (25-Sep-2007): Duration against PvP targets has been reduced to 10 seconds.
Patch 2.0.1 (05-Dec-2006): In PvP, Crowd Control effects will last no longer than 12 seconds instead of the full duration, with a chance of a heartbeat resist. Casting a lower rank polymorph on a target that is already under the effect of a higher rank will cause an error message and not consume mana.
How it should function:
Patch 1.2.0 (18-Dec-2004): Players now have an increasing chance to break free of the effect, such that it is unlikely the effect will last more than 15 seconds.
Currently I haven't seen any CC last longer than 10 seconds, and am seeing mage polymorphs last 10/10/9 seconds (1st/2nd/3rd cast within DR time frame). Long duration CC needs to have the bell curve adjusted a bit.