Blizzard is once again changing the way in which we trigger heroic modes of encounters. First there was Sarth, where you left up mini-bosses. Then there was Ulduar, where you (sometimes) triggered a particular event by defeating or leaving up particular bosses or objects. Then in the Trial of the Crusader you had four different raids to do depending on your difficulty setting.
Now in Icecrown Citadel, Blizzard has appeared to reach a final solution of sorts -- potentially even going back and changing the way hard modes in Trial of the Crusader are triggered. Difficulty (hard mode on/hard mode off) of the upcoming Icecrown Citadel raid in patch 3.3 will be set on a per-boss basis.
The full statement after the break.