Ghostcrawler has spoken up in this thread a little bit about the balance of PvE and PvP play that we'll be seeing in the next expansion. Specifically, he says that they're slowing the pace of PvP play down a little bit: "Health pools will be much larger in Cataclysm and healing will be lower." We'll have more health overall, and healing spells will heal less. That will make a difference in PvE as well -- it'll take more hits for the boss to finish off the tank, so heal size is expected not to matter as much as healing coordination. It won't matter if your tank is up to full health as much as it matters whether or not you can help them survive the next hit.
Obviously, this is very early theory for an expansion that's still in development. But what GC is saying is that Blizzard wants healers to strategize a little bit more about which heals they're using rather than depend on big healing to get the job done. Avoidance will matter, he says, because it'll help conserve healer mana. Fights will be more about strategy than overpowering with big heals.
Cataclysm's endgame is a long time away, but that's Blizzard's idea for it right now.