Think about it.
Tier 1, Naxx-Sarth-EoE - 2 difficulties - 200-226.
Tier 2, Ulduar - 4 difficulties (if you include 10/25m hardmodes) - 219-239
Tier 3, ToC + ToGC - 4 difficulties - iLevel 232 - 272 (Yes, there is iLevel 272 loot from the tribute chest on 25man)
Tier 4, (speculation), Icecrown - 4 difficulties (they have said that there will be hardmodes, if that means heroic modes or ulduar-hard modes that remains to be seen) - iLevel 248 - 284?
But will we see iLevel 284? It seems like a huge scale to go through 84 iLevels though one expansion, but yet it seems like that is gonna be the progression...
They better not make this puggable...