The model that Blizzard's looking to move towards is getting a test run of sorts in Icecrown Citadel -- lower tank avoidance, bosses hitting for more reasonable amounts, and -- in Cataclysm -- higher health overall. In other words, we'll be healing for roughly the same amount we're healing now through less damage that nonetheless occurs more frequently. This will lead to a greater risk of running OOM as the encounter progresses if you dump a ton of mana on damage that should have been healed more efficiently. Even now, there are encounters like hard-mode Vezax where you do have to do this, which I thought made for a pretty interesting fight.
But while I think it's a good solution to both of the current "Oh crap the tank died in 2.7 seconds" and "Blanket the raid in Rejuvenation and never hit 40% mana" problems, I have a lurking suspicion that it's going to make hard-mode raiding even uglier than it can be now. One of the consistent features of current Wrath hard modes is the degree to which the heal team can be made utterly miserable (Firefighter, 3-tree Freya, heroic Twins-25, phase 3 heroic Anub'arak-25), and that's without much risk of going OOM. Introducing that element when Blizzard is so fond of encounters with everyone in the raid taking hideous damage worries me more than a little.