You could easily fix this for Cataclysm. How? Get rid of the RNG from avoidance.
Instead of dodging 100% of an enemy's damage 30% of the time, make it so that tanks dodge 30% of an enemy's damage 100% of the time.
In addition, it seems more logical that a tank could cause a "glancing" type blow rather than dodge an enemy completely, especially in all that armor!
In this manner, a tank would get hit by a boss 100% of the time, but he would reduce the damage taken by a combination of his armor, dodge, parry, and block (for paladins/warriors).
This would completely eliminate "spiking" and allow healers to have more strategy in terms of using more mana efficient heals, not always spamming their biggest heal just so that the tank doesn't get two shot.
On average, there wouldn't even be a huge change in damage taken, because a tank's 40% avoidance would just reduce 100% of the damage he takes by 40%, rather than taking 100% of the damage 60% of the time, and 0% damage 40% of the time.
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This would be a huge change in game mechanics, but I think it is a necessary one if you want to resolve this dilemma permanently.
It would give you a lot more freedom in terms of encounter design, designing the combat ratings for level 85, and even the way that healing would play out in cataclysm (in terms of both heal numbers and mana costs).